MONIAN SYSTEM OF ROCKS. 503 



come up as far as Llanerchymedd, and all the rocks that I have 

 examined which are not granite are very clean gneisses of felspar, 

 quartz, and white mica, for the most part beautifully foliated, but 

 in one instance, in which the rock has a felsitic aspect macroscopically, 

 entirely without foliation, though otherwise of exactly the same 

 structure. Others not microscopically examined have a more slaty 

 or politic appearance, and one, at least, is composed of such granitic 

 materials as to suggest derivation from granite. Yet at the only spot 

 where the contact is seen it has a thoroughly intrusive appearance 

 (see fig. 13). The granite itself, which is best seen at Pen-Ion, and 



Pig. 13. — Section in Quarry east of Pen-Ion. 





1. Granite. 2. Schistose micaceous rock, 



at Llaneiddog Bach, is different from that in any other district, being 

 nearly pure white, of fine grain, and abundantly supplied with 

 crystals of white mica. It has a very slight tendency to foliation ; 

 even this, however, is subject to variation, and the above description 

 applies best to the variety at Llaneiddog Bach. Possibly this may 

 be an older mass than any other, composed of the same material 

 which supplied the grey gneiss, before the dioritic eruptions had 

 commenced. It is remarkable that this district is one of the very 

 few Pre-Cambrian ones in which lead-ores occur, though these are 

 probably of Post-Ordovician date. 



The Eastern District. 



In spite of its easy accessibility this district has been sadly 

 neglected. In many respects it is the m,ost interesting, and in some 

 respects certainly the most difl&cult of all to understand. Neither 

 Sir A. Ramsay nor Dr. Callaway tell us much about it, and there is 

 much more to tell. 



Origin op the Hornblende-schists. — For the right interpretation 

 of the larger part of the area the first necessity is to come to a conclu- 



