Immediately north of Fredericktown are a few small granite mounds. One is really in the northwest part of the village on the west bank of Saline creek. It is only a few rods in diameter. Another similar exposure is just north of the railroad station, and two others exist about one-half mile to the east. All the rock carries small amounts of biotite, has red feldspar, and is tolerably fine-grained. Two and one-half miles northwest of Fredericktown is a low granite hill (Tp. 33 N., R. VI E., Secs. 1 and 2). The surface exposure covers about forty acres. It is a coarse-grained, red and black mottled variety.

West of Fredericktown, about two miles, on the eastern side of Buckner mountain, is a long, narrow strip of granite, which lies between the porphyry, forming the hill and the limestone surrounding it. It is a red rock with a small amount of black mica. In places it is coarse-grained, but toward the west gradually passes into the porphyry. On the western side of the same elevation the porphyry often becomes sufficiently well crystalized to admit of being worked into paving blocks.

South of Fredericktown several small outcrops of granite occur. There is also apparently a ridge extending southward, which is composed of granite, but has not, as yet, been exhumed through erosion. The summit was encountered in excavating a cellar on the Calhoun place. A little farther southwest, near the river, a granite knoll occupying an area of a couple of acres is found.

Another small area of granite is found on the south side of mount Devon. On the northwestern spur of Matthews mountain (Tp. 33 N., R. VI E., Sec. 34), is an irregular mass of granite covering some 50 acres. Bowlders of decay, 20 feet in diameter, are found here. The rock is the ordinary red variety of granite. On three sides of the mass the granite gradually changes into porphyry.

The eastern base of Hawkin mountain is composed of granite. It is a narrow strip extending northwest and southeast for a distance of fully one and one half miles. At the southern end it gradually passes into porphyry. The typical rock is well crystallized, and at some points, especially near

the Wesley chapel, contains an unusually large amount of black mica. The feldspathic constituent is red, forming a red and black mottled rock.

A mile to the southwest, on a mound just beyond the extreme eastern portion of Tin mountain, is a similar mass of granite. Toward the west it changes into porphyry.

The

The headwaters of Cedar creek pass between two small granite hills. The easternmost occupies several acres. other is larger and higher. The rock is a typical granite-porphyry; but in its fracture, in its weathering into immense rounded bowlders and in its general appearance, it resembles typical granite. On the eastern side of the hill it becomes eminently porphyritic.

East of Blue mountain is a prominent, very steep, rounded hill, on the southwestern flank of which is a small area of granite, of much interest on account of its isolated position in a broad field of porphyry. It is a gray variety, but with only a very small amount of black mica. In texture, it is fully as coarse-grained in the central part as the ordinary granite, but gradually becomes finer toward the margins.

PORPHYRY.

Physical Characters-The porphyries are hard flinty rocks, generally of a reddish color. The latter, however, ranges from a light pink to a dull, dark gray or purple. In texture there is considerable variation. The groundmass is dense and very fine-grained. It assumes all graduations from a coarse microgranite to a fine devitrified glass. Through the groundmass are scattered abundantly large individuals of quartz and feldspar, the phenocrysts. The fine-grained, compact character of the rock enables it to resist degredational influences in a remarkable way, as is shown by the fragments loosened in jointing, which preserve sharply all their irregularities long after the hardest granite bowlders have become perfectly rounded or entirely decayed. The rugged topographic forms presented also clearly indicate the same properties of withstanding

weathering influences. The fracture of the porphyry is splintery or subvitreous.

Chemical Composition-In the porphyries which have been analyzed there is not much variation in the acidity, rarely more than 4 per cent, the range being from 68.60 to 72.40 per cent. An analysis by Melville, of the typical porphyry, the samples of which were obtained just beyond the west boundary line of the sheet (Tp.33 N., R. V. E., Sec. 5), gave the following results:

Specimens taken from the summit of a high granite knob that occurs two miles northwest of the Silver mines, the top of which is capped by porphyry, yielding the following results to the chemist of the St. Louis Sampling and Testing Works:

Mineralogical Constitution-What has already been said regarding the mineralogical composition of the granite applies also to the porphyry. The chief constituents are the feldspar and quartz, but the difference in structure of the rock gives them very different relationships. The accessories zircon, magnetite and apatite, are found scattered through the groundmass. The ferro-magnesian ingredients are not, as a usual thing, well developed. The feldspars are the same as those of the granites, and comprise the four varieties: orthoclase,

microcline, albite and oligoclase. They range from grains of microscopic dimensions in the groundmass to large phenocrysts, an inch or two in length. The porphyritic crystals of quartz are frequently rounded with the characteristic em bay. ments, due to partial remelting before the original solidification of the mass.

Microscopical Structure- The most striking differences between the granite and porphyry are shown when thin sections of the two rocks are examined under the microscope. The fine-grained matrix, in which are embedded the larger crystals, at once characterizes the latter. The groundmass varies considerably from the typical microgranitic structure to an almost glassy one which, however, is usually so thoroughly devitrified that its original character can hardly be recognized. The various phases which are especially noticeable, are the microgranitic, granophyric, micropegmatitic, felsophyric, vitrophyric and spherulitic. A particularly interesting facies is the trachytic, in which the minute crystals of lath-shaped feldspars

FIGURE 12 Magmatic corrosion of

quartz.

FIGURE 13. Quartz phenocrysts.

are arranged in the same way as in diabase. The phenocrysts are mainly quartz, though those of feldspar are not of infre quent occurrence. The appearance, under the microscope, of the porphyritic quartzes which have been rounded or given irregular outlines as the result of partial resorbtion, is represented above (figure 12). Some of the larger quartz crystals, however, preserve their crystallographic faces, and in thin sec tions give sharp, even outlines (figure 13).

Genetic Relations of the Acid Rocks-The fact that no sharp line of separation exists between the porphyry and granite, and that everywhere the two kinds of rock merge gradually into each other, the transition zone in different places varying from a dozen to forty or a hundred yards in width or depth, appears to afford good evidence that the rock masses represent, not necessarily numberless different eruptions as was once believed, but comparatively few with special differentation in the different parts. Since the structural character of massive rocks is dependent largely upon the accidental physical conditions under which the molten magma cooled and solidified, it is of prime importance to take into account the fundamental principles involved when considering the relations of the two rocks. Under ordinary circumstances, the texture of an igneous mass is glassy, or very fine-grained at the surface. Its grain becomes coarser as the distance from the surface increases. Granite is commonly regarded as an abysmal rock, one which has cooled very slowly and under great pressure. Porphyry is formed when the pressure has not been so enormous, and the cooling has been retarded less. At the surface, where solidification has gone on very rapidly and the pressure has been virtually wanting, amorphous glass is the result. All these stages may exist in the same mass, but secondary changes usually have obscured or obliterated the original features.

In the Mine la Motte district the conditions just described appear to have prevailed. Within the granitic area, where high, steep sided hills exist, the coarse-grained granite graduates upward into fine-grained varieties and finally into typical, aphanitic porphyry, which serves as a protecting cap to the elevations. Similar graduations upward of the granite into the porphyry in the isolated mountains are clearly observable in a number of cases, and it seems probable that these facts would hold true in most instances, if proper means of observation were afforded.

Within the limits of the sheet the principal granite masses clearly appear to be areas in which the superior surface facies of the acid magma had been removed through erosion; and the