MONIAN SYSTEM OF KOCKS, 



505 



gneiss becomes nearly horizontal, and finally 

 turns up and becomes darker at the end 

 of the cutting. Now this intrusive rock 

 is of similar structure to the patches we 

 have been considering further west — with 

 the same needles of chloritized hornblende 

 and the same epidote-spots, and in part it 

 is foliated and in part it is not. A similar 

 section is seen in the cutting between 

 Gaerwen and Holland Arms. 



Erom these facts I can see no escape. 

 The rock which thus behaves carries with 

 it the igneous origin of all the rocks in 

 the neighbourhood which are of similar 

 character, and which surrounding circum- 

 stances prove may be intrusive. 



In particular it must be mentioned that 

 the rock which is coloured as " greenstone " 

 on the Survey map, and runs in a narrow 

 band from Tanhirion to Plasgwyn Lodge, 

 is of the same character, as is that on the 

 summit of Mynydd Llwyddiart and in the 

 other bands in those hills, which run, so far 

 as I have been able to trace them, irregu- 

 larly among the gneisses. 



The Gneisses and Mica-schists. — The 

 grey gneiss of the Eastern has essentially 

 the same characters as that of the Central 

 District, and there can be no hesitation in 

 correlating them. Nevertheless the mica 

 in the former is on a larger scale, it is 

 less constantly parallel, and there are often 

 layers of crystalline granules apparently 

 derivative. On the large scale we find 

 three distinct varieties, dependent on the 

 circumstances in which the rock is found. 

 The normal type has the planes of foliation 

 regular and distinct, and forms an easily 

 recognizable brownish rock. It is often 

 banded in more or less siliceous layers, and 

 is subject to contortion without losing its 

 character. This form is most prevalent 

 on the eastern side. Secondly, when in 

 the neighbourhood of the diorite, it may 

 be compacted into a mass with the ex- 

 ternal aspect of a felsite, though its internal 

 character is un obliterated. Such a mass 

 is seen within a few feet of the diorite at 

 Y-graig, and in actual contact with it in 



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