Page images
PDF
EPUB

GEOLOGY.

THE ARCHEAN ROCKS-THE PALEOZOIC ROCKS-ECONOMIC GEOLOGY.

As has already been stated, the rocks of the Iron Mountain sheet consist of crystalline, massive Archean rocks, of crystalline stratified Algonkian rocks, and of clastic, stratified paleozoic limestones and sandstones. Beginning with the oldest, these will now be described in succession.

THE ARCHEAN ROCKS.

By Erasmus Haworth.

The Archean rocks of the area covered by the Iron Mountain sheet are eruptive or crystalline rocks, which may be conveniently divided into two general classes: (1) The Basic Eruptives, which occur in the form of dikes; and (2) the Acid Eruptives, including the granites and porphyries.

THE BASIC ERUPTIVES OR DIKE ROCKS.

The rocks of this class are found principally in the southeastern part of the sheet. They usually occur in well-marked dikes cutting through. the granite and porphyries, or in boulders resulting from a partial surface decomposition of the same; but in a few cases they constitute bosses almost circular in outline. The dikes vary in width from less than half an inch to more than 150 feet. They usually trend northeast and southwest, or nearly so, but in some instances they vary widely from this. The walls are generally nearly vertical, and when otherwise, seem to be so irregular that no general law can be stated governing them.

On the map the locations of the dike rocks are shown. In many cases the limits of the dike itself could not be defined on account of the covering of soil and other products of decay. But in such cases numerous boulders are found lying on the surface.

Throughout the whole country the various dike rocks are remarkably similar, and differ so much from all other rocks to be found that they can be readily distinguished.. They are dark in color, sometimes nearly black, but often shading into a dark green, while the other rocks are a light green, or some shade of red or brown. They are, when altered by decomposition, typical samples of what many geologists formerly called "green-stone." They are also much heavier than the ordinary granites or porphyries, and when in long thin fragments, will ring like metal if struck with a hammer.

Mineralogical Composition of Dike Rocks.-In the well-crystallized and unaltered specimens, the principal minerals of these rocks are the following: Triclinic feldspars, generally more than one, augite and olivine, with iron oxide and apatite as constant accessories, and brown mica as an occasional accessory. In those which are less perfectly crystallized there is always present a varying amount of unindividualized, or glassy matter, with the same minerals mentioned above; in such cases the feldspar crystals are small and slender, the augite crystals rarely well formed, and the olivine, in many cases, wholly wanting; but the oxide of iron is a constant accessory. If the rock is much weathered, the augite and often portions of the glass are changed into green, fibrous hornblende, which gives a green appearance to the rock. If the weathering is carried farther the feldspars are partially changed to kaolin, and here and there through the mass scales of white mica are observed.

Texture of Dike Rocks.-In texture these rocks vary from the holocrystalline, as coarse-grained as the ordinary granite, to a very compact glassy rock. In many of the smaller dikes, and occasionally in the larger, the microscope shows that more than half the volume of the rock is glass the lava having cooled so rapidly that the particles of matter did not have opportunity to arrange themselves in crystals. Quite often certain mineral crystals, formed before the lava completely solidified, are scattered here and there through the rock, and producing what is known as the porphyritic texture. In other cases the texture of the rock is uniform throughout, and is quite coarsely crystallized

Descriptions of Dikes -Thirteen different dikes are represented on the accompanying map, though the presence of some of them was judged of only by boulders lying on the surface; doubtless many more exist. In the following descriptions they will be taken up consecutively, beginning in the north.

1. At Iron Mountain, in a number of different places, mining operations have revealed dikes cutting through the porphyry. Some of these are less than 12 inches wide, while others are two or three times as large. On the west side of the hill such may be seen in contact with the overlying Paleozoic rocks, showing that they were formed prior to Paleozoic times. At the summit of the hill where the excavations are deepest, large masses of impure hornblende are found in contact with the ore. These probably represent dikes which have broken

through the porphyry, possibly at the time when the fissures originated in which the large ore veins were formed.

.

2. Along the main road leading from Pilot Knob to Farmington, about 50 yards south of Brewer's creek, in section 15, township 34 N., 4 E., boulders of dike rock were seen. The rock is quite finegrained, is of a dark bluish-green color, is very tough and hard to break, and fragments of it ring like metal when struck with a hammer. In weathering, a very smooth surface is produced.

3. In section 24, township 34 N., 4 E., on land belonging to Mr. Elijah Smith, is a large dike, the rock of which, near the surface at least, is largely composed of long slender crystals of hornblende. Portions of this rock have also yielded green fibrous hornblende or uralite.

4. On the south side of a little porphyry hill in section 8, township 33 N., 4 E., are numerous boulders of dike rock, indicating the presence of a dike somewhat larger than the average. The rock is much coarser grained than the one last described, has a less greenish tint, and the boulders are roughened on weathering, due to the existence here and there through the rock of portions which resist decay better than the remainder of the rock.

5. In the southern part of section 3, township 33 N., 4 E., is an unusually steep, circular, porphyry hill which stands out alone in the valley. Adjacent to its southwestern side a narrow, sharp ridge extends to the southeast for nearly a fourth of a mile, which is so narrow and sharp that it has been named locally the "Hog-back." This ridge is a dike of a very fine-grained, glassy rock. The ridge owes its existence to the great resistance to erosion offered by the dike rock, the limestone on either side of it having yielded much more rapidly. The surface covering conceals the contact between the limestone and the dike, and thus their relative ages could not be absolutely determined, though there is little doubt but that the latter is the older.

6. Just south of Bowman's field, in the north half of section 6, township 33 N., 5 E., a few boulders of a dike rock were found which are of more than passing interest on account of their containing porphyritic quartz crystals. The quartz grains vary from microscopic dimensions to as much as an eighth of an inch in diameter. They are always greatly corroded, apparently as though after their formation they were partially redissolved, and as if solidification arrested the operation before it was finally completed.

7. On the north bank of Stout's creek, about half a mile above its mouth, just north of McAllum's field, near the center of the north half of section 4, township 33 N., 5 E., is a mass of dike-rock boulders, indicating the existence of a large dike beneath the surface. The boulders are large and sound, and yield fine specimens of olivine diabase.

8. Just south of the house formerly occupied by Mr. John Burk, in south half of section 4, township 33 N. 5 E., is a dike which can be seen cutting through the granite in the bed of a little branch. It is from three to four feet wide, and can be traced for about 20 feet. It trends northeast and southwest. The rock composing it is very fine-grained, black in color, and has a sub-conchoidal fracture. About two rods below this a second but much smaller dike crosses the branch in a direction parallel to the first. It is composed of essentially the same materials.

9. On the north bank of Rock creek, near Mr. Richard Payne's house, in section 17, township 33 N., 5 E., is a large dike, the exact width of which could not be determined on account of the covering of surface detrital material. Numerous large boulders are to be seen, and a little digging, done years ago, in time of an excitement about tin ore, showed that the solid dike was near at hand. At this place, also, small amounts of an inferior grade of asbestos were found in the surface clay. It has evidently some connection with the dike, but until found in place no prediction can be made regarding its extent. The rock at this place is quite coarse-grained, some of the slender feldspar crystals being a half inch or more in length.

10. Along the upper portions of a ravine running east, in the northeastern portion of section 20, township 33 N., 5 E., a dike may be seen traversing the porphyry. The rock is much lighter in color than the ordinary dike rock, and, judging from its mineralogical composition as revealed by the microscope, it is doubtless much less basic. It has a very fine texture, and macroscopically seems to be a homogeneous mass, but in reality has many minute feldspar crystals scattered through it.

11. On the northwest side of a high porphyry hill near the northeast corner of section 30, township 33 N., 5 E., numerous small dike-rock boulders were found. They were of the ordinary fine-grained kind and of no especial interest.

12. Along a small ravine running northeast, near the center of section 29, a dike is exposed in the porphyry. It has been strongly fractured, and each fissure has been filled with quartz. The fissures with their quartz veins pass into the porphyry, showing that they have been formed since the dike rock solidified. The rock itself is finegrained, and possesses no qualities which would particularly distinguish it from other small dike rocks.

13. On a hillside half a mile west of Hogan, about a hundred yards north of the road, numerous boulders of dike rock cover the ground. They are olivine diabase, well crystallized, and are similar to that so extensively worked into paving blocks at Skrainka, in Madison county.

14. In a diamond drill hole in the limestone, just west of Shepherd mountain, the St. Louis Ore and Steel Co. at one time struck a dike rock at a depth of about 250 feet, approximately at the same depth at which porphyry was found in the other places near by. It is a beautiful steel-gray rock, composed principally of white feldspar and dark augite, the crystals of each mineral sometimes being one-half inch in length. When polished it is very handsome, and would make an unusually fine ornamental stone if it could be had in sufficient quantity and of proper dimensions.1

THE GRANITES.

The granites of this area are very important, both from a commer. cial and a scientific standpoint.

Mineralogical Composition of Granites.-The constituent minerals are principally quartz and feldspar, with the microscopic accessories, such as rutile hairs in the quartz and zircon crystals in the feldspar. The quartz is the ordinary variety, usually without distinct outline, but occasionally with a few partially-developed crystallographic faces. The feldspars include the two potash feldspars, orthoclase and microcline, and also oligoclase, with possibly small amounts of andesine. In a few places black mica occurs, but is never evenly disseminated; the segregations vary in size from that of a bird's egg to three or more feet in diameter, and are a great hindrance to the quarrying of the rock for such portions are of little value. Only in the rarest cases has hornblende been found, and then only as a few scattered crystals. In

1 This dike is not represented on the map because it has no surface exposure.

« PreviousContinue »