MONIAN SYSTEM OF ROCKS. 475 



the island, and tho absence of Cambrian rocks in the trough-faults 

 -close at hand, points to their being of Ordovician age, and they thus 

 supply a proof of the absence of true Cambrian deposits in this 

 area. I expect it must be continuations of these that occur in 

 the skerries, which I have not been able to visit, but which are 

 marked on the Survey Map as " hard conglomerates and grits." 



In the portions of the Western District now described there are 

 three classes of rock, which from their wide distribution in the 

 island, and their special and characteristic nature, I have reserved 

 for particular description. They all occur isolated, and are due to 

 actions which are limited to a small area. These are : — 



1. Epidote Infusions. — Certain parts, both of the chloritic schists 

 and of the higher rocks, are found to be more indurated, and to weather 

 into round outstanding knobs, or portions of beds. They differ from 

 tho surrounding mass by being dark green in colour and finely 

 crystalline in fracture, but without orientation of the crystals. 

 These, on examination, turn out to be full of minute crystals of 

 epidote scattered uniformly through the rock. I can think of no 

 better explanation of them than that the rock has been locally 

 saturated with some mineral water, the reaction of which on the 

 surrounding material has produced the epidote crystals, and hence I 

 call them " epidote infusions." They are seen at Forth Delise, 

 north of the river Alaw, north of Porth-y-defaid, at Portk-y-corwgl, 

 at Berth Wen, Roscolyn, and elsewhere. 



2. Quartz Knobs. — These are the most remarkable features of all 

 the Anglesey rooks, and their origin is a matter of great difficulty. 

 In their characteristic form they stand up as isolated hummocks on 

 the surface of the country, and are often nearly as high as they are 

 long or broad. They may be merely low mounds, or may rise as 

 high as a house, or form a good-sized hill. Usually they are 

 elliptical in outline, but may be narrow and long, though it is 

 doubtful whether we should refer the latter to the same category, 

 or consider them as ordinary veins. In structure they may possess 

 scarcely any clastic elements, or they may contain many very 

 rounded pebbles of pure quartz, the whole or the remainder being 

 composed of clear quartz in dusty-lookiug polygons of growth 

 separated by clear lines, the dusty appearance being due to exces- 

 sively fine cavities, the largest of which may possess fluid-enclosures. 

 The only impurity is a little occasional sericite which may form 

 round the larger pebbles where the matrix is not quite close. The 

 purity and structure of the rock, together with its mode of occur- 

 rence, forces me to the conclusion that it has been formed on the 

 spot where it is now found, though the pebbles may have been 

 brought there along with the formative material. They are not 

 veins in form or in structure, but they may have had a similar 

 origin. The only suggestion I can ofi'er is that they are the result ' 

 of the cooling of hot water which has bubbled up and eaten away 

 the rock into a cavity, then deposited quartz on the sides, in some 

 cases has broken up again the first deposits, and rounded the 



