ON THE OLDER BOCKS OF ANGLESEY. 397 



may be conveniently divided into five groups: — 1. The granitic rocks 

 and their associates. 2. The volcanic gronp east of Paiys Mountain. 

 3. The dioritic rocks and their allies. 4. The gabbros and serpentines. 

 5. The isolated masses and dykes. 



1. The Granitic Tiochs and their Associates. — These rocks are imme- 

 diately distinguished from the sedimentary rocks of similar composition — 

 i.e., the mica schists and gneisses — by the largeness of their elements. They 

 seem indeed quite another order of rock ; instead of the numerous minute 

 elements of the schists, a single crystal of felspar will often occupy more 

 than the whole field of view, and though the rock is holocrystalline there 

 are none of the sutural junctions so characteristic of the schists. The 

 only orientation in any of them is a very rough linear one, not observable 

 nnder the microscope. If the larger size of the resulting elements is 

 taken to indicate a greater intensity of metamorphism, then these granites 

 must have been subjected to such influences to a point far beyond the 

 gneisses. The orientation of the latter indicates that the forces they were 

 subject to were ' directed,' but its absence in the granites indicates agencies 

 without direction. These conditions are satisfied only by the production 

 of a mobile magma. In other words, the rocks must have been melted. 

 The only evidence of any previous state consists in the occurrence of such 

 minerals as quartz, mica, and felspar as enclosures in the various elements 

 which now form the rock. 



The largest mass of granite occurs in the Central district, where it 

 shows several types. We may divide them roughly into those in which 

 quartz is a considerable element, and those in which there is little or 

 none. Of the first of these divisions the freshest examined is that at 

 Henblas, Llandrygarn (117). In this the felspar elements are very large, 

 and the greater proportion are plagioclase. They are very dusty, and in 

 some cases are so much altered as never to completely extinguish ; in 

 other cases they are scarcely altered at all. There are some brown mica 

 elements, whose position and relations indicate that they are of the same 

 order of consolidation as the felspar. The quartz is more continuous, and 

 encloses areas of felspar, so that it plays the role of groundmass, though 

 some of its elements are large. Fresh, however, as the rock appears, it 

 has abundant signs of having been submitted to pressure since its con- 

 solidation. The quartz often breaks up into smaller elements, showing 

 an imperfect microspectral polarisation ; the larger elements polarise 

 spectrally, and the appearances of microcline are seen in some of the fel- 

 spars. If this rock be compared with the granite of Killiney, the resem- 

 blance is remarkable. The latter is certainly fresher, but it has the same 

 relations of the elements, and shows also spectral polarisation and micro- 

 cline structure, but the quartz is not so broken up into smaller elements. 

 On the whole, it appears to only require subjection to a little more pressure 

 to become identical. The best preserved granites at Llanfaelog (112) 

 and Craig-yr-allor (116) are practically the same rocks, and in some parts 

 of the former the quartz is unbroken, and the resemblance of these parts 

 to the Killiney granite is almost complete. On the whole, however, the 

 quartz has been broken by cracks, and these have become broadened and 

 filled with mica, now passed over into chlorite, so that the aspect of the ulti- 

 mate product is very different from the original. The great majority of 

 the granites examined, however, are thoroughly cataclastic rocks — as, for 

 example, the tongue of that rock at Porth-y-ly-wod (113), at Gwalchmai 

 (115), at BrynTwrog further north (120) and at Tnys Coed, Coedana( 121). 



