404 BEPOKT— 1888. 



the hornblende is -well crystallised. A second rock from the northern 

 boss (132) has the change carried still further and combined with the 

 chemical, the whole rock being an irregular mass of calcite and chlorite, 

 with stray fragments of unaltered felspar and hornblende, to show 

 whence it has been derived. 



The rocks which lie to the north of Llandrygarn, though well supplied 

 with hornblende, appear to belong to a different group, more nearly allied 

 to the granites. Two of these, one near Llandrygarn (13:^), and the 

 other near Penterfyn (134), are very much alike. In these the felspars 

 are in much larger elements, and often contain apatite, and there are 

 isolated areas of quartz. The hornblende, mica, ilmenite, and sphene 

 which make up the rock crystallise only in the intervals, and have there- 

 fore what may be called negative outlines. A similar type of rock occurs 

 in the valley south of Llecheynfarwy. The band of diorite which ex- 

 tends northwards to Plas Llanfihangel has been examined near the 

 latter spot. It represents the Craig-yr-allor types in rather larger ele- 

 ments. In the rock to the south-west of the farm (136) the horn- 

 blende and felspar are about equal in amount. The latter is much 

 sericitised and contains a little apatite, and the former has in many places 

 pyroxenic patches taking the place of a part or the whole of the crystal. 

 There are also some dark brown rutiles. The rock to the east of the farm 

 (137) is one of the most complex in the island. It has a kind of ground- 

 mass composed of a complex of small and very irregular elements of 

 quartz and felspar, the latter being the most abundant and often occun-ing 

 in large sericitised plates and containing apatite. The hornblende is in 

 large patches, each of a single crystal, generally decomposed along the 

 cleavage lines, and without definite boundaries. There are several well- 

 preserved patches of augite, some of large size, and not in this case con- 

 nected with the hornblende ; there is a considerable amount of the usual 

 sphene; and, finally, there are large ophitic masses of the ordinary garnet. 

 Some short, dusty, high refracting pieces may even represent decomposed 

 rutile. If this rock commenced as an oi-dinai-y dolerite, chemical meta- 

 morphosis could hardly go much further. 



It is remarkable that at Pen-bryn-yr-Eglwys we have, in a small band, 

 the two varieties which are seen to occur in the Central district ; one of 

 these (38) is the coarse form, such as occurs north of Llandrygarn. It 

 has a groundmass of well-banded plagioclase in large elements, with little 

 crystals of apatite; and the hornblendic element, which is in irregular, len- 

 ticular, and somewhat orientated bands, is now entirely converted into 

 chlorite ; the mica also which was present is mostly now in the same 

 form ; but there is no quartz. The other (39) corresponds entirely with 

 the finer rocks at Craig-yr-allor. There is the same uniformity of size 

 and distribution of the small elements ; the felspar is apatite-bearing, 

 there is the same unaltered ilmenite ; but the hornblende, readily recog- 

 nised by its cleavages and general build, has now, as in the last case, been 

 entirely changed to chlorite. 



When we put these observations together, and call to mind the 

 various alterations traceable in these rocks, both mechanical and chemi- 

 cal, which succeeded the presence of hornblende within them, we may 

 be certain that, if this hornblende has ever been derived from an augite, 

 the change must have taken place at a very early period, and under quite 

 different conditions to those which have brought about the later changes. 



In the Eastern district there is an important mass of diorite in the 



