390 EEPOKT— 1888. 



structure seems to indicate a sedimentary rock whose materials were 

 obtained from other deposits in the neighbourhood. The rock at Cerrig 

 Ceiuweu (94) consists of a mixture of fragments of calcite and jasper, 

 re-cemented in a mass of calcite. These fragments are irregular and 

 unworn, so that, whatever was their first origin, they have been broken up 

 in situ — i.e., the rock is cataclastic. Its structure is beautifully shown by 

 the paraboloid. 



With these limestones may be included certain rocks which owe their 

 origin to an infiltration of calcareous matter into rocks already formed. 

 A very considerable number of the ordinary rocks have a certain amount 

 of calcite in veins, proving the presence of such material in the infiltrating 

 water ; but only in two cases amongst the rocks observed has this gone 

 so far as to make the calcitic element predominate. One of these occurs 

 in the laminated schists at Porth-y-defaid (IG), and gradually passes into 

 the ordinary rock in the direction of the larainas. The groundmass is 

 quartz in large elements, and the calcite is imperfectly crystallised in 

 minute particles which aggregate together in irregular areas. There is 

 also here a quantity of chalcopyrite, which decomposes into a homogeneous 

 transparent substance with the colour of malachite. The other is in the 

 fine-grained rocks at Chlorach Bach, near Llanerchymedd (103), where 

 a quantity of calcareous matter without crystalline form is mixed up 

 uniformly with a mass of minute elements of quartz and sericite. It 

 diSers only from the other rocks of the neighbourhood by the abundance 

 of the calcareous matter. 



Quartz Knohs. — The rocks to be described nnder this head all occur as 

 isolated knobs of greater or less size, surrounded on all sides by shales 

 or schists, in whose orientation they take no part and produce no inter- 

 ference. They have, in fact, no orientation, either on the large or on the 

 microscopic scale, and they show no signs of contortion. Hence their 

 stratigraphical relations in no way suggest, but, on the contrary, strongly 

 oppose, the idea of their being brought into their present position by the 

 folding of any bed. They occur in various places and in various rela- 

 tions, yet they have always approximately the same form. These facts 

 must be borne in mind when we attempt to interpret their structure. 

 They are for the most part entirely composed of quartz ; only one or 

 two, which must be referred to the same group, contain a trace of other 

 minerals. By this character they are completely cut ofi" from all the 

 other rocks of Anglesey — even the whitest quartzite in a bedded form, 

 such as that at Porth-y-gwalch, being much less pure. We may consider, 

 first, those in which no certainly derivative elements can be recognised ; 

 and then those in which they are certainly present. 



Of the first group the foremost place must be given to the great knob 

 to the south-west of the Parys Mountain (232) (see fig. 18). This is for the 

 most part composed of large quartz elements, comparable with those of a 

 granite, which from their intimate sutural boundaries may be certainly 

 judged to be authigenetic. These are dotted over with very minute white 

 specks, which are almost absent from their boundaries, so that, as seen 

 with the paraboloid, the rock is divided into a number of closely-fitting 

 polygons. This pohjgonal structure, which is very characteristic of 

 these quartz knobs, is to be seen also in many of the veins and patches of 

 secondary quartz in the ordinary sedimentary rocks, and hence may be 

 taken to be an additional evidence of authigenesis. 



There are also occasional irregular patches of smaller elements. In 



