ON THE OLDER ROCKS OF ANGLESEY. 389 



and a th.ird at Ceryg Moellon, are closely associated with tlie serpentine 

 of that area, and will be described in connectioa with the latter. The 

 one at Ceryg Moelion has been considered by Professor Bonney to be an 

 infiltration from carboniferous limestone which may once have overspread 

 the district. 



Another type occurs amongst the unstratified materials, either where 

 the rocks are fine-grained, or where they are obviously volcanic accumu- 

 lations. These are generally formed of small isodiametric crystals, 

 without twinning, which form a mosaic, and only contain the large 

 twinned crystals in later cracks. They are exceedingly pure, and show 

 no signs whatever of pressure, and occurring as they do in lenticular 

 patches it is impossible to believe that they are parts of bedded rocks 

 brought into their present position by folding ; they must, on the con- 

 trary, have crystallised in situ. Those with the largest crystals, about 

 •■002 inch diameter, occur where the rocks are most altered, as behind the 

 Druid Inn on the old Holyhead Road (92) in the Centi'al, and at Rhyd 

 Eilian (202) in the Eastern, district. A similar rock has been described 

 by Professor Bonney from Wugan, Pentreath. Where the rocks are of 

 finer grain, as at Llanfaethlu (26), the calcitic, or perhaps dolomitic, 

 elements are smaller ; but the smallest of all, being not more than "0002 

 inch diameter, are found in the limestone at Llanlliana (218), where the 

 rock is on a larger scale, and is more or less stratified. In those masses 

 which are found amidst volcanic accumulations, and whose mode of 

 occurrence is similar to that of an intrusive rock, there is generally some 

 admixture of quartz. Thus at Port Unal, on the Northern coast (215), 

 the general elements are small dolomitic (?) ones, but the shi'inkage 

 cracks are filled with quartz ; and at Careg Gwladys (205) quartz 

 occupies narrow interstices between the calcareous elements, as well as 

 the cracks, and sometimes increases so much as to form the greater part 

 of the rock. 



Another remarkable type, but of somewhat similar mode of occur- 

 rence, is the oolitic rock of Llanbadrig (217) (see fig. 19). The original 

 basis of this is a crystalline mass of small elements with shrinkage areas 

 of larger ones, and is therefore identical with the Llanfaethlu limestone. 

 But this original mass has been broken up into fragments, each fragment 

 has been coated with a band of calcitic dust, and then these dust-coated 

 fragments are enclosed in a similar matrix to the original. Two or three 

 thus imbedded go to form a larger fragment, and this again is coated with 

 another layer of calcitic dust, or a second coat may be deposited on an 

 original fragment, and this process goes on again and again ; and, lastly, 

 the interstices between the final fragments have been filled up with cal- 

 cite in larger elements. We can thus trace the gradual building up of 

 the rock in situ, and can recognise that the process is exactly what might 

 take place in the action of a calcareous spring, producing a chemical 

 deposit. 



Certain limestones in the series have a more bedded aspect in the 

 field, and these are found to differ fundamentally in their structure from 

 the above. Thus another rock at Careg Gwladys (204), which looks 

 more coarsely crystalline, has a number of large, well-formed rhombohe- 

 dral crystals of dolomite, either isolated or in groups, imbedded in a 

 matrix of quartzose fragments interspersed with a brownish opaque dust. 

 The edges of the dolomite crystals generally have a band of clearer 

 crystalline matter, but they are often broken and irregular. Such a 



