Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/298



I should not however conclude this account of the minerals which occur in this part of Glen Tilt, without mentioning that fibrous limestone is also found in some of the rifts, showing a little of that pearly and changeable lustre, for which the satin spar is so remarkable.

I have thought it unnecessary in describing either these or the other rocks which I have examined, to particularize the several anomalous mixtures of substances which are found about them, since they are extremely partial and generally limited to the vicinity of the junctions. They all appear to be of a mechanical nature, or at least to have resulted from the same causes which produced the several disturbances already described, and they offer no particular instruction, while at the same time they can scarcely be defined by words.

Quitting these marble beds and descending still the course of the Tilt, schist, quartz rock, and limestone, are seen alternating for about 500 yards. In one place a bed occurs in the quartz rock, forming a regular part of the series, but composed of the constituents of granite, these being sometimes disposed in a granitic manner, and sometimes possessing the foliated texture of gneiss. This is followed by a schist, which seems intermediate between gneiss and micaceous schist, having the aspect of the latter with its shining surface and even foliated structure, but shewing in the cross fracture the grains of felspar which belong to the former.

Proceeding further on, a great series of these thin beds may be observed, of which some on being broken present the aspect of true granite, inasmuch as the mica is irregularly placed and the laminæ of the rock, however foliated in position, are by no means foliated in structure. A fragment so broken as to be divested of its external flat form, would be considered as a fragment of granite