t^90 Dr. Mac Culloch on the Geology of Gkn Tilt. 



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 vici- 

 nity 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 con- 

 stituents 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 

 laminae 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 



