Dr. Mac Cull ocH on the Geology of Glen Tilt. 291 



since it must be esteemed an essential character of gneiss that the 

 mica should possess an arrangement, at least so far parallel, as to 

 give the appearance of a foliated structure. 



As whatever view we can take of this occurrence is followed by- 

 important consequences, it is necessary to consider it somewhat 

 more particularly, and I shall first examine that case in which the 

 gneiss is found alternating with the schist. We have already seen 

 in numerous instances, that great anomalies take place in the aspect 

 of the schistose rocks, as well as of the limestone, wherever they 

 approximate to the granite and are traversed by granite veins, I 

 must therefore premise that no such veins are seen in this place. 

 The mass of granite is here indeed removed to a considerable dis- 

 tance, and the schistose beds are perfectly straight and undisturbed. 

 The general series consists of mica slate, alternating in a very 

 irregular manner with hard argillaceous schist and quartz rock, and 

 with a few beds of limestone. With these beds is found that mi- 

 caceous schist which contains felspar, and which will probably 

 be considered by most observers as a modification of gneiss. If 

 then it be called gneiss, that rock alternates with quartz rock, clav 

 slate, mica slate, and limestone, an anomaly at variance with the 

 almost universally received doctrines respecting precedence in rocks. 

 If we now turn our attention to those specimens which have the gra- 

 nitic aspect, I may remark that, however decided this character may 

 be in §ome particular points about this place, I shall here lay no 

 stress ofi it, as better examples can be adduced from the hills at no 

 great distance. Although the granite, in the Instances which I shall 

 enumerate, occupies but a small space among the lamlnse, its texture 

 is perfectly granitic, and bears not the slightest resemblance to that 

 of gneiss, since the mica Is placed in all directions, and the portion 

 in question, if detached from the neighbouring parts, could not be 



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