ox THK OLDER EOCKS OF ANGLESEY. 401 



basic in composition. They contain no quartz, but are principally com- 

 posed of large crystals of plagioclase, with some possibly original mica. 

 Tt is, however, irregularly cracked, and the cracks are filled with rai()a 

 and with a certain amount of hornblende of apparently secondary origin. 

 This is closely associated with the rocks of dioritic type, which, as else- 

 where in the island, are of a schistose character, apparently owing to the 

 tendency of the hornblende to orientation. In one example examined 

 (249) this mineral is very abundant, and is certainly of the same gene- 

 ration as the accompanying plagioclase, both being clearly crystallised 

 and of equal importance. There is also abundance of isolated, and there- 

 fore probably original, brown mica, Quartz occurs sparingly, both as 

 separate elements and as small inclusions, in the mica. The accessory 

 minerals are numerous and important, including apatite and ilmenite, 

 and a few crystals of sphene. Another example (251) contains tho 

 largest crystal of sphene seen anywhere in Anglesey. All the elements 

 of these rocks are smaller than those of the granites. It is difficult to 

 be certain of the origin of these schistose diorites in this locality, their 

 stratigraphical relations being obscure. They are probably unconnected 

 with the granites and later in date. Their basic character, their accessory 

 minerals, and their association, all speak against their being developed 

 from the neighbouring sedimentary rocks, as supposed by Mr. Allport. 

 They show no signs of pressure. 



We can in fact observe what may well bo considered as the true sedi- 

 mentaries in the immediate neighbourhood (250), and these are of an 

 entirely different character. They are slightly orientated, and are com- 

 posed of rather rounded elements of quartz, mica, and felspar, much of 

 the latter being plagioclase ; and in general they closely resemble the 

 gneisses of other districts. It is probable that these are the rocks which 

 formed the ground out of which the volcanic products were erupted. 



The small-grained rocks of the district are exceedingly various, and 

 it seems almost impossible to classify them, each one examined being 

 different to the others. Moreover, their structure is utterly irregular, 

 being nothing but a confused mixture of various minerals; according 

 to the collocation of these minerals so is the rock. Very few, however, 

 show any signs of pressure. In one group there is much secondary mica. 

 Of these there is a beautiful example at Abertywedog (239), full of dark 

 mica, interspersed with ferrite, and containing also quartz in irregular 

 patches and sericitised felspar. Its chief peculiarity, however, is its 

 containing the beautiful decomposed garnets, already described, some of 

 which are large enough to occupy the whole field of view. In another 

 of the same micaceous type, at Penrhyn-glas (246), and which also con- 

 tains some ilmenite, the decomposed garnets are so abundant as to almost 

 play the role of groundmass to the other elements. The others of this 

 group take their character from the principal second ingredient. Thus 

 at Llanwenllwyfo (242) we have a micaceous rock, in which the principal 

 mineral is quartz, with apatite and brilliant zircons. In one south of 

 Nebo (247) the principal mineral is sericitised felspar, with apatite and 

 ilmenite ; and in one at Dulas (237) the quartz and felspar are about 

 equal in amount. One of the rocks of this group, at Forth Lygan (235), 

 exceptionally shows feeble signs of foliation. It has abundant brown 

 mica, with sericitised felspar, small scattered elements of quartz, with 

 ilmenite or leucoxene, and another mineral, now utterly altered and 

 decomposed, but which occasionallv shows the cleavages of hornblende. 



1888. ' D D 



