ON THE OLDER ROCKS OF ANGLESEY. 403 



faults, so that we have little opportunity of comparing them witli neigh- 

 bouring sedimentary types. One, however, of this class, from the northern 

 side at Ty Newydd (245), has been examined, and is seen to differ essen- 

 tially in structure, and correspond to the more altered gneisses. It con- 

 sists of larger elements, bounded by dark lines, probably due to decay, 

 of rather rounded outline and closely fitting. The larger ones are fel- 

 spar, and the smaller quartz, and orientation is produced by abundance of 

 brown mica. It contains also a lapillus of a rock, consisting of a mixture 

 of quartz and sericitised felspar in small elements. This so closely resembles 

 in general structure the supposed sedimentary rock from the east of 

 Parys Mountain, that the conclusion is strengthened that these are the 

 true sedimentaries, and the group of rocks above described are of special 

 and volcanic origin. 



3. The D'ioritic Bods and their Allies. — The Dioritic type of rocks, 

 besides those described in connection with the last group, occur in large 

 quantities in the Central and Eastern districts, and to a small extent, in 

 connection with the granite, in the north-west. 



In the Central district there are various patches between Craig-yr-allor 

 and Plas Llanfihangel. At Craig-yr-allor itself (130) the rock is very rich 

 in hornblende, and consequently has a dark-green colour. The crystal . 

 line elements are small throughout, and individually are very irregularly 

 ari'anged. The rock, however, in hand specimens shows some confused 

 orientation. The felspathic elements, which are now entirely converted 

 into sericite, play the role of groundmass, and in these there are dusty 

 patches of the decomposed and not yet recrystallised felspar, in one of 

 which the twinning lines may still be seen. The hornblende is scattered 

 over this without crystal outlines, and the rock is very rich in sphene in 

 small crystals and patches, with an occasional nucleus of ilmenite. In 

 certain spots there is a pyroxene, which occurs in a similar manner to 

 the hornblende, but less conspicuously. In one case it appears to form 

 part of a hornblende crystal. It would appear, therefore, not improbable 

 tliat the rock was originally a dolerite, in which each mineral has changed 

 to its appropriate substitute, and the whole become somewhat schistose 

 in the process. In another rock from the neighbourhood (131) there is 

 a more conspicuous banding by the irregular segregation of the horn- 

 blende and felspar. The latter is often scarcely altered, but may be 

 traced gradually becoming sericitised. The former is very clean, and 

 the only signs of pyroxene are some apparent inclusions in certain of the 

 crystals. In its other features the rock agrees with the above. The rock 

 on the summit of the craig itself (128) is a much finer-grained rock, and 

 is only very obscurely orientated. Here the small, rather rounded felspar 

 elements are very clearly seen, and the hornblende is clearly crystallised, 

 and the titaniferous element is represented entirely by ilmenite. It 

 would appear, therefore, that the rock is less altered as a whole than the 

 first described. Yet, if it be really derived from a dolerite, it is remark- 

 able that the only sign of pyroxene occurs in minute corners and specks 

 at the edges of the hornblende. As far as the evidence of these rocks 

 themselves may go, they show no sign of disturbance since their first 

 consolidation, any alterations observable or suggested being entirely 

 chemical. But in another rock in the district (129) the whole has been 

 broken into fragments, the larger ones lying in a cement of the smaller 

 ones, making a true mortar-structure. Yet the large fragments are very 

 clean, the felspar is well preserved and contains inclusions of apatite, and 



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