retain their crystallographic faces. The outlines of one is shown in the following cut (figure 15), in which also a glassy core is present in the crystal.

FIGURE 15. Augite Phenocryst, with glassy core

Location of principal dikes-The length and extent of the dikes are known in but few instances. Some of them which lie close together may, perhaps, belong to the same intrusive The dikes are much more numerous in the southeastern part of the central granite area than elsewhere.

mass.

On Butler hill, west of Knob Lick (Tp. 34 N., R. VI E., Sec. 8, NW. qr.), occur numerous bowlders of dike rock. They trend northeast and southwest. Farther down the hill. side to the northeast, a dike three feet wide, cuts through the granite. The two, doubtless, are parts of the same dike. To the northeast, a short distance, in a small quarry, a dike six inches wide is exposed in the granite.

Still farther to the east, bowlders of diabase appear at a number of places indicating the existence of dikes, but none are visible until a small quarry is reached on the northeast side of the mound, about one-half mile south of the Milne and Gordon quarry. Here is an excellent example of a small black dike cutting through red granite. Fragments composed in part of each of the two types may be obtained, the dike rock adhering to the granite with great tenacity. A fourth dike occurs in a quarry on the south hill face. The character of the rock is the same as the others in the area.

Two and one-half miles due south of Knob Lick station there a number of dikes whose locations are indicated by surface bowlders (Tp. 34 N., R. VI E., Sec. 21). They are found on the east side of the highway leading to Fredericktown. In three different places, each about 200 yards apart, the bowlders are found in great abundance. The south one seems to be the largest. The line of bowlders may be traced for some distance northeast from the road. As some of the bowlders are quite large, the dike is certainly three to four feet wide. The similarity of the rocks in the dikes indicates an intimate genetic relation. They are not very coarsely crystallized, and are composed of diabase-porphyrite.

At Rocky glen, on Rock creek, two miles north of the Mine la Motte dressing works, is an isolated dike. The stream passes between two sandstone hills. For a distance of 40 rods it has worn its channel through the sandstone to the crystallines. In a number of different places small dikes may be seen in the granite. At the point where the hills come closest together is a dike about forty inches wide which passes in under the sandstone. It is quite evident that the sandstone was formed long enough after the intrusion of the dike for the surface to be worn down to a level with the granite. In the bed of the creek, on the north side of a small granite mound east of the dressing works at Mine la Motte, a dike of diabase porphyrite is exposed. The surface covering entirely conceals it both to the north and south, so that it can be traced only a few feet. It is not over five or six feet wide, and it trends northeast and southwest.

Two miles west of Mine la Motte station (Tp. 34 N., R. VI E., Sec. 31), on the north side of a hill by the road is a small outcrop of dike rock, which is a good example of olivine diabase. The exposed area is small, and is occupied partly by bowlders, and partly by rocks rising three to four feet above the surface of the ground. These are portions of the dike which have resisted decay better than the adjoining country rock. The basic mass is very solid and compact, is coarsely

crystalline throughout, and is practically identical with the dike rock at Skrainka.

At Skrainka occurs a dike which is widely known on account of a paving block quarry which has been open in it. For years it has been worked for paving material which is sent into the market under the name of "blue granite." The quarry is located at the largest exposure of the dike. There are other outcrops near by which probably belong to the same intrusive. mass. The rock is coarsely crystallized, of a bluish gray color, and is as typical an olivine diabase as there is in the region. The quarry is located on the southwest side of the hill, but the dike extends up a ravine to the east several hundred yards, and up a second small ravine to the northwest fully as far. The hill to the north is porphyry, which gradually changes into granite; that to the south and also the one to the west are of similar character. West of Skrainka less than a mile is another exposure of dike rock, which has been worked into paving blocks to a limited extent. Still further west is a second exposure of bowlders covering several acres of almost level ground. The dike itself is not visible. Across the ravine to the west, the presence of a third dike is shown by the occur. ence of numerous bowlders.

On the south slope of Skrainka hill, in a ravine running southwest, is a dike five feet wide, cutting porphyry. It trends north 30 degrees east, and has a number of small branches leading off from it nearly at right angles. It is very finegrained, so much so that macroscopically it seems to be a perfectly homogeneous mass.

Along the Fredericktown and Ironton road, from three to five miles west of the former place, large masses of dike rock are exposed at numerous places. Some are badly decayed, while in the case of others there are large, fresh bowlders. A similar exposure also occurs a short distance south of the road near Frizzell branch. The character of these rock is the same as at Skrainka, so much so that specimens from the two places cannot be distinguished from each other.

On the north slope of Mount Devon, in the bed of a ravine opening into the Little St. Francois river, just above an old ford, is a dike seven feet in width. The characters of the rock are peculiar. About one foot of the mass on each side is the ordinary diabase, but the remaining portion is entirely different. Large crystals of a light pink feldspar are scattered through the rock, some of them being fully two inches in diameter.

Another dike of coarsely crystallized diabase is located three miles to the southwest, in a branch of Matthew creek (Tp. 33 N., R. VI E., Sec. 28); and one also a little further to the southwest in section 31.

One of the largest diabase masses in the whole region is on the northwestern flank of Tin mountain (Tp. 33 N., R. VI E., Sec. 30). It is of particular interest on account of a great excitement which arose more than 20 years ago, concerning the supposed existence of tin ore. The dike is fully 50 yards wide at one place, and appears to cut into the northern side of the hill; but, as it cannot be found either to the north or south, it is probable that it is a boss, rather than a dike. The rock differs from that in and around Skrainka in being much less perfectly crystallized, and in having a darker color and a more waxy lustre. The microscope shows that fully one-fourth of the mass has never crystallized and, therefore, exists in the form of a glass.

East of Silver mines, on Pine creek, bowlders of diabase indicate the proximity of a dike several feet in width.

At and around the Silver mines are a number of dikes. On the left bank of the St. Francois river, just below the stone dam, a dike four feet wide cuts through the granite. It has yielded to decay rapidly so that it appears as a stair-case with granite walls. The rock is very fine-grained, dark-colored and contains much glassy matter. Across the stream another

dike may be seen.

Back from the river on each side bowlders are quite common, although the dikes are not visible. At one place on the west side, about one hundred yards from the stone dam, bowlders