412 EEPOKT— 1888. 



A peculiar rock at Gwalclimai (138), which unfortunately is not seen 

 in a weathered state but only in a quarry in the village, has essentially 

 the same general structure as the above group, but it looks quite black, 

 and like an earthy pelite. It consists entirely of rather larger microliths, 

 crowded together and overlapping, and which also appear from their 

 extinction angle to be oligoclase. They are set in a matrix which, 

 contains nothing but black and red ferric dust, and a non-polarising base. 

 It might almost be called a basalt but for the absence of any magnesian 

 constituent. Perhaps andesite would be the best name to apply 

 generally to these rocks. The interest of this black one lies in the fact 

 that it is the only one of its kind yet found in situ in Anglesey, whereas 

 numerous fragments with a similar structure occur in the overlying 

 rocks near Bangor and elsewhere. 



The only remaining rocks whose structure the microscope is of 

 value in elucidating, are the diabases. There are, of course, many 

 such rocks in Anglesey of more recent date, but those now to be 

 described are entirely conBned to the oldest series. One of the most 

 interesting is that near Llyn Trefwll, which lies between the chlori- 

 tic schists and the great Ordovician beach breccias, and with which 

 the small mass of granite already described is associated. Some of 

 this is schistose, some is not. A non-schistose example (56) consists 

 of a dusty, but otherwise transparent, base, in which are numerous 

 felspar microliths as well as black specks, which may be magnetite ; it is 

 dai-k between crossed Nicols, but this is partly due to the development 

 of a non-polarising chloritic substance. In this base are numerous insets 

 of augite, many of the smaller of which are idiomorphic. There are also 

 some patches of dark earthy matter, apparently included fragments. The 

 rock is therefore a typical diabase. The schistose variety (57) differs in no 

 essential respect from this, except that the augite is smaller, is scarcely 

 ever idiomorphic, and looks fragmentary. The apparent schistosity is 

 due solely to a few nearly parallel cracks, and an almost imperceptible 

 orientation of some of the microliths. Apparently continuous with this 

 rock is a green mass which contains fragments of granite (58). This 

 green rock, however, is really a diorite like that of the Central district, 

 rich in hornblende, with a little apatite. It is, therefore, probably 

 unconnected with the diabase, and forms part of the breccia bound 

 to it by pressure. This is the more probable, as the fragments of 

 granite described by Professor Bonney were associated with a slaty 

 matrix. In the neighbourhood of Dinas Llwyd, and northwards to 

 Bodowen, there are numerous lenticular patches of igneous rock, which, 

 unlike the later dykes, are orientated witb the sedimentary rocks of the 

 district, here largely of volcanic origin. One of these has recently been 

 described by Mr. Barker (' Geol. Mag.' N.S. Dec. iii. vol. v. p. 267) as an 

 olivine diabase. The specimen examined for this report, from nearer 

 Dinas Llwyd (127), would be rightly described by that title. It does not, 

 however, show tbe ophitic character of the augite, which is described by 

 Mr. Harker, but, like his specimen, it contains large masses of ilmenite, 

 a characteristic mineral of the older but not of the newer rocks. It is 

 probable for all reasons that this belongs to the older group, and in this 

 case it is the only one in which olivine has been recognised. A 

 mylonitic form of apparently the same type of rock occurs a little west 

 of Llangwyfen (125), in the same district. It has drawn-out crystals 

 of augite, plagioclase, and perhaps of diallage, in the midst of lines of 



