3. On a New Method of Measuring Crystals, and its Application to the Measurement of the Octahedron Angle of Potash Alum and Ammonia Alum. By H. A. MIERS, M.A., F.G.S.

The two fundamental laws of crystallography-namely, (1) the constancy of the angle in crystals of the same substance, and (2) the law of simple rational indicesseem to be violated by those crystals which are liable to irregular variations in their angles, or those which have the simple faces replaced by complicated 'vicinal' planes.

Both these anomalies are exhibited by potash and ammonia alum. Brilliant and apparently perfect octahedra of these salts show large variations in the octahedron angle; other crystals show low vicinal planes in place of the octahedron faces.

If it be true, as is supposed, that the octahedron angle varies in different crystals, it would be interesting to ascertain whether progressive variations can be traced during the growth of a single crystal, and whether some or all of the octahedron faces change their direction in space if the crystal be held fixed during growth.

In order to solve this problem a new goniometer has been constructed, in which the crystal is fixed at the lower end of a vertical axis, so that it can be immersed in a liquid during measurement.

This device is in reality an inversion of the ordinary goniometer with horizontal disc; the liquid is contained in a rectangular glass trough with parallel-plate sides; one side is placed rigidly perpendicular to the fixed collimator, and the other is perpendicular to the telescope, which is set at 90° to the collimator. The trough is supported on a table which can be raised and lowered, so that the crystal can be placed at any required depth in the liquid. If the liquid used be its own concentrated solution the crystal can be measured during growth, and the changes of angle, if any, can be observed at different stages.

In order that it may be held rigidly, the crystal is mounted, when small, in a platinum clip, which it envelops as it grows larger.

The results derived from the measurement of a large number of alum crystals are as follow:—

(1) The faces of the regular octahedron are never developed upon alum growing from aqueous solution.

(2) The reflecting planes (which are often very perfect) are those of a very flat triangular pyramid (triakis octahedron) which overlies each octahedron face.

(3) The three faces of this triangular pyramid may be very unequal in size. (4) The triakis octahedron which replaces one octahedron face may be different from that which replaces another octahedron face upon the same crystal.

(5) During the growth of the crystal the reflecting planes change their mutual inclinations; the triakis octahedron becomes in general more acute, i.e., deviates further from the octahedron which it replaces, as the crystal grows.

(6) This change takes place not continuously, but per saltum, each reflecting plane becoming replaced by another which is inclined at a small angle (generally about three minutes) to it.

(7) During growth the faces are always those of triakis octahedra; if, owing to rise of temperature, re-solution begins to take place, faces of icositetrahedra are developed.

Conclusions.

The above observations prove that the growth of an alum crystal expresses an ever-changing condition of equilibrium between the crystal and the mother liquor. It does not take place by the deposition of parallel plane layers; new faces are constantly developed: since these succeed one another per saltum they doubtless obey the law of rational indices, though not that of simple rational indices.

From the mutual inclinations of these vicinal faces it is possible to calculate with absolute accuracy the angle of the faces to which they symmetrically approximate. This angle is found to be that of the regular octahedron, 70° 313. The

octahedron angle of alum is not, therefore, as appeared from the observations of Pfaff and Brauns, subject to any variation.

The angle at which a given vicinal plane is inclined to the octahedron face is independent of the area of the plane, and of the temperature of the solution and of the barometric pressure: it appears to be conditioned by the concentration of the solution at the surface of the plane.

In confirmation of this view it is found that the upper and lower portions of an octahedron face which stands vertical are often replaced by two different triangular pyramids; also that the three faces of one such pyramid are at a given moment not necessarily equally inclined to the octahedron face which it replaces.

When, as is often the case, one of the three vicinal planes is large and the other two are too small to give a visible reflexion, the face appears to be a single reflecting plane. It is this which has been mistaken for the octahedron face in previous observations.

Similar phenomena of growth are exhibited by crystals of other substances belonging to different systems. The conditions of equilibrium between the crystal and the solution are such that vicinal planes appear in place of simple forms; these vary with the concentration of the solution, and give rise to variations in the measured angles which are only apparently anomalous. Their true position can be determined on a crystal of cubic symmetry (such as alum) whose theoretical angles are known.

A further study of the faces developed during the growth of crystals will, it is hoped, lead to a better understanding of the reasons why a simple face like the octahedron should not be a surface of equilibrium, and of the relation between the vicinal planes and the structure of the crystal.

4. A Comparison of the Pebbles in the Trias of Budleigh Salterton and of Cannock Chase. By Professor T. G. BONNEY, D.Sc., LL.D., F.R.S.

The pebbles in the two deposits correspond in certain respects. In both vein quartz and various quartzites are abundant, with certain dark green rocks, which undoubtedly in some, probably in most, cases owe their colour to minute tourmaline. The compact quartzites so common in Staffordshire are found in the Devonshire deposit, but less abundantly, while other quartzites more rare in the former are commoner in the latter. A quartz-felspar grit like the Torridon Sandstone occurs in both. The following are points of difference. The shape of the Staffordshire pebbles is nearer to a prolate spheroid, that of the Devonshire to an oblate one: igneous rocks are much rarer in the latter, and the compact tourmaline rocks in the former. The author thinks that the pebbles at Budleigh Salterton must have travelled from a more or less south-westerly quarter, and that the frequent correspondence of materials indicates that somewhat similar detrital rocks fringed the archæan rocks of the ancient western land in localities far apart.

5. On a Soda Felspar Rock at Dinas Head, North Coast of Cornwall. By HOWARD Fox.

Dinas Head adjoins Trevose Head, four miles west of Padstow.

The base and foreshore of the headland appear to be entirely composed of greenstone containing much calcite, probably an altered dolerite.

Between the greenstone and the slate, as well as interbedded with the slate, occurs a rock which covers about an acre. It assumes various characters, all of which contain nearly 10 per cent. of goda and from 64.4 to 666 of silica. The compact varieties are crypto-crystalline, and might easily be mistaken for cherts. The concretionary and spherulitic varieties show grains and blades of a felspar which is doubtless albite. It varies in colour from creamy grey to light brown and dark bluish-grey; it weathers white, and is often studded with cavities filled with

rusty brown material containing crystallised quartz. This rusty-coloured material occasionally weathers out as nodules 8-10 mm. in diameter, projecting 10-15 mm. beyond the white surface of the rock. In other places this rock contains lenticles and concretions of calcareous matter of considerable size, with concentric structures around some of them. When interbedded with slate it is densely studded with small rusty brown spots, sometimes irregular and sometimes of such forms as would be yielded by rhombs; they are undoubtedly pseudomorphs after carbonate. It has in places all the appearance of a stratified rock, and is distinctly bedded. In one locality this rock assumes a nodular form, with the outer edges composed of spherulites varying from 2 to 10 mm. in diameter. The central portions of the spherules are composed of crypto-crystalline material, the outer portions of radiating blades or prisms of felspar, presumably albite. Ferric oxide is scattered through the rock in irregular patches, in veins, or in radial streaks between the blades of felspar, or occasionally it is almost wholly concentrated in the polygonal sutures formed by the mutual interference of adjacent spherulites.

The greenstone is posterior to and intrusive in this soda rock, and cuts across, bends, and disorders its beds. Extreme crushing has in places altered the original junction line, and thrust planes and fault breccias are seen. Quartz veins traverse both rocks, but are more numerous in the soda rock.

The slate is interbedded on the northern side of the headland with bands of blue limestone, and is occasionally studded with ferruginous patches and nodules in much the same way as the soda rock.

A rock containing as much as 7:54 per cent of soda is said by Kayser to occur as a contact product due to greenstone in the Hartz. The question arises, Is this Dinas Head rock an adinole or a soda felsite-i.e., a keratophyre? The presence of ferriferous carbonates favours the theory of its being an altered sedimentary rock, whilst the spherulitic and concretionary structure favours the theory of its being an igneous rock.

6. Report of the Committee on Geological Photographs.
See Reports, p. 274.

SATURDAY, AUGUST 11.

The following Reports and Papers were read :—

1. Report of the Committee on Palæozoic Phyllopoda.-See Reports, p. 271.

2. Report of the Committee on the Eurypterid-bearing Deposits of the Pentland Hills.-See Reports, p. 302.

3. Preliminary Note on a New Fossil Fish from the Upper Old Red Sandstone of Elginshire. By R. H. TRAQUAIR, M.D., F.R.S.

These remains consisted of large broad thick plates gently hollowed in boatlike fashion, and showing no articular surfaces along any of their free margins. None of these plates had been found entire, though pieces had occurred of over a foot in length and a quarter of an inch in thickness. To these plates the author has given the name of Megalaspis Taylori.

4. On the Homes and Migrations of the Earliest Forms of Animal Life as indicated by Recent Researches. By HENRY HICKS, M.D., F.R.S.,

F.G.S.

The author, after giving a history of the finding of zones of animal life at lower and lower horizons in the Cambrian rocks during the past forty years, referred to some recent evidence which points to the extension of similar forms of life over very large areas. That there were many centres of dispersion in different parts of the world seems certain; but as the migrations of the forms found on each side of the Atlantic seem to have taken place contemporaneously, the author believes that the original home of these forms must have been at some point in the Atlantic when that basin was much narrower than it is at present. The author then referred to the evidence, showing a gradual development in these earlier forms of life, and to some points bearing on the question of evolution.

5. On some Vertebrate Remains from the Rhætic Strata of Britain. (Third Contribution.) By MONTAGU BROWNE, F.G.S., F.Z.S.

LABYRINTHODONTIA.

Parts of jaws and teeth of a Labyrinthodont Amphibian are found commonly, though very imperfect, in the bone-bed' of Aust Cliff, Gloucestershire. So much material has been accumulated that it may be as well to record that ten specimens of the pre-maxilla-showing large 'tusks' and a serial outer row of smaller teethare known to the writer, four of which are in the Bristol Museum, where they are labelled 'Jaws of Enaliosaurians,' and six others, from Aust and Westbury-onSevern, are in his own possession. One of these was sent as the 'scute of a reptile ; ' and another, before its development from the matrix, was so much like a coprolite as to lead to the inference that others may have been previously passed over. Of the maxilla there are six large portions and several smaller ones, carrying both large and small teeth of two distinct characters. Several large and many small pieces represent the mandible, some, near the symphysis, showing large teeth. Of teeth, both seated upon the bone and broken away, there are a great many examples of all sizes. Five pieces are apparently portions of the palato-vomerine element, carrying large and small teeth.

Between fifty and sixty specimens are fragments of the jaws, and there are many specimens which are doubtless portions of the elements of the skull and of the thoracic plates. A few portions of limb-bones are doubtfully referable to the Labyrinthodontia.

The definite determination of the pre-maxilla, portions of the maxilla, mandible, and some other parts of the Labyrinthodontia, appears to be a new record for the Rhætic of Britain. No speculation is hazarded as yet as to the generic or specific determination of these remains, as if not referable to Metoposaurus (Metopias) diagnosticus,' they appear to have affinities with Trematosaurus in the character of the united or single pre-maxilla, and by the large tusks being internal to the serial mandibular teeth; on the other hand, the anterior or 'tusk' teeth are large-some of them in. in diameter-each with a correspondingly large pulpcavity, and although apparently simply plicated in the exserted portion, yet at the extreme base the plications of the dentine are complex, and somewhat resemble those shown in the teeth of Mastodonsaurus Jägeri as figured by Meyer.2

The presence of teeth of two characters-large or Labyrinthodont, and small or Saurichthyan-in the same jaws, together with the characters of the external surface and alveolar palatal extension of the maxilla and of sections thereof, leads to the conclusion, hinted at in the last Report under the heading of Termatosaurus crocodilinus, that 'Saurichthys' is a non-existent piscine genus, and that jaws Miall, Rep. Brit. Assoc., 1874, p. 157; but see Lydekker, Cat. Foss. Rep. and Amph., p. 157.

3

Die Saurier des Muschelkalkes (1847–55), Tab. 64, fig. 2. • Montagu Browne, Rep. Brit. Assoc., 1893, p. 749.

1894.

UU

figured under that heading are mainly referable to those of Labyrinthodonts, and that teeth of Saurichthys' of various authors can be definitely assigned—

1. To Labyrinthodontia, sp.

2. To Plesiosaurus rostratus.

3. To Hybodus, sp. (symphysial teeth).

4. To Gyrolepis, sp., and perhaps Colobodus, sp.

'RYSOSTEUS.'1

This genus was instituted by Owen from the examination of a single anterior dorsal vertebra half imbedded in its pyritic matrix from the bone-bed of Aust Passage, near Bristol,' and of other portions from Westbury-on-Severn; the type specimen not being in existence has apparently prevented any reference to similar specimens since that time, and it may be as well to put on record that some few nearly perfect examples of dorsal and other vertebræ, and many portions of others, of the character of those attributed to Rysosteus Oweni, have been procured by the writer from Aust Cliff and Westbury-on-Severn, Gloucestershire.

Whether Rysosteus be truly a reptile is not mooted at present, until the whole of the material in the writer's possession has been exhaustively examined; but, from the striated character of the neural and hæmal spines, and the characters of the centra of the vertebræ, &c., the resemblance to some amphibian such as Urocordylus Wandesfordi is by no means incomplete.

DINOSAURIA.

Phalangeals similar to those from Aust Cliff in the British Museum, and attributed to Zanclodon (?), are in the writer's possession, but a fairly large vertebra, 2 in. in length by 2 in. in height, exclusive of the neural spine, which is missing, may pertain to Zanclodon, or is perhaps more closely allied to Massospondylus; in either case this vertebra will be a new record for Britain. Limb-bones, both large and small, are also in his possession, and are provisionally assigned to this order.

6. On some Forms of Saurian Footprints from the Cheshire Trias. By OSMUND W. JEFFS.

The picturesque quarries in the Lower Keuper (building stones' series), situated at Storeton Hill in Cheshire, have long been familiar to geologists as being the scene of the earliest discovery in England of the famous Cheirotherium footprints by Messrs. Cunningham and Yates in 1839. Besides the impressions of Cheirotherium and Rhyncosaurus (figured by Mr. G. H. Morton, F.G.S., in his Geology of the Country around Liverpool') these quarries yield impressions made by other species of animals, which have been obtained by the author during several years' study of the district. Some of these forms (hitherto undescribed) were exhibited. They include

(a) Genus non det.-Slab of Keuper sandstone showing hind and fors feet of a smaller animal than C. Stortonense, with narrower toes, which curve inwards and are not separated, nor do they radiate as in true species of Cheirotherium.

(b) Genus non det.-Tracks of a small animal inch in length, with a stubby foot and having very distinct claws on the digits.

(c) Genus non det.-A still more minute form, inch in length, showing four digits tapering to a point, with no vestige of claws.

(d) An oval impression, with concave terminated digits and a hinder projecting spur.' Toes webbed. This may be the impression of a chelonian.

R. Owen, Rep. Brit. Assoc., 1841 (1842), pp. 159, 160; J. Morris, Cat. Brit. Foss, 1854, p. 353.

2 A. S. Woodward and C. D. Sherborn, Cat. Brit. Foss. Vert, 1890, p. 282. T. H. Huxley and E. Perceval Wright, Trans. Roy. Irish Acad., 1867, pp. 359-362, pl. xx.

The paper will be published in extenso in the Geological Magazine.