402 KEPOET— 1888. 



This may be compared with tlie hornblendic granite already described. In 

 another rock near the same spot (236), but close to a crystalline mass, 

 the whole mass has been broken up into fragments or torn out into 

 strings. The best interpretation which can be suggested for these is that 

 they are irregular masses composed of the materials of a granite in the 

 form of volcanic debris. 



A non-micaceous group has some quartzose representatives, as at 

 Abertywedog (240), where the other elements are fairly preserved plagio- 

 clase and calcite, with a little leucoxene and apatite ; and also some which 

 are quartzless, as to the south of Nebo (248), where the rock consists 

 of an irregular aggregation of plagioclase and calcite without definite 

 boundaries. It also contains a considerable quantity of sphene, and a 

 small amount of apatite and garnet. 



Besides the above, which are composed more or less of granitic ele- 

 ments, there is associated with them in the same complex, and apparently 

 of the same general age, a group of rocks which in some cases is certainly, 

 and in others probably, pyroxenic. The clearest of these, at Rhos Manarch, 

 inland (243), may almost be called a dolerite, though the general arrange- 

 ment of its elements is very like the irregular rocks described above. It 

 contains abundant augite, in broadish plates of irregular eroded outline, 

 and also, to judge by the cleavage, similar plates of diallage, associated 

 in an indefinite way with unhanded felspar, together with small sphenes 

 and garnet. It is full of crystalline calcite, and has a skeleton crystal 

 of pyrites. The former of these may well be secondary, as a similar 

 mineral fills the numerous cracks. Of the others, owing to their state of 

 decomposition, it is not so easy to be certain. One at Trwyn (241), 

 vrhich forms a beautiful object with the paraboloid and is somewhat 

 orientated, has a groundmass of sericitic material, which from its arrange- 

 ment may be the result of the entire decomposition of some felspar ; with 

 this are long patches of minute and feebly polarising elements, not unlike 

 serpentine, which are also probably the result of entire decomposition of a 

 second mineral. These spaces are scored with parallel cracks, now filled 

 -with calcite, whose crystals on either side eat into the adjoining spaces. 

 Thirdly, there are some opaque areas of brownish dust, crossed by lines 

 which appear to indicate a cleavage such as that of augite ; and, lastly, 

 there are many sphenes. Though utterly decomposed this appears to 

 have been a holocrystalline rock of felspar and two pyroxenes. 



There are two other rocks in this district which it does not appear 

 possible at present to classify with certainty, though both are well-defined 

 in their peculiarities. One of these, at Abertywedog (238), has a ground- 

 mass of garnet, much clear quartz in rather rounded elements, with 

 inclusions of zircon and apatite, irregular patches of calcite, and crystals 

 of decomposed sphene with brownish sub-opaque patches like worn 

 crystals, with numerous parallel lines of brown dust, which seem to indi- 

 cate a well-marked cleavage, such as that of diallage. In this the quartz, 

 like the garnet and the calcite, may be secondary and adventitious, but 

 at present, if the dusty material be diallage, it has not the associates yve 

 should expect. The other, from Forth Lygan (234), consists of uniform, 

 small, rounded elements, apparently of felspar, with the narrow crevices 

 filled with slightly orientated sericite. From the occurrence of insets in 

 a similar rock at Pant-yr-Eglwys we know that this must be an altered 

 felsite, but it looks more like one composed of minute lapilli. 



This complex of various rocks is for the most part bounded by 



