MONIAN SYSTEM OF ROCKS. 495 



In some of these cases we have clear intrusive junctions, in others 

 the junction is indefinite. The cause of this difference seems to 

 depend partly on the nature of the neighbour and partly on its 

 relation to the mass of the granite, which in the latter case had 

 the power of absorption, but not in the former. 



Such is the positive evidence that the granite of this district is 

 intrusive. If it is insufficient, I can have no conception of what 

 would suffice. If it is accepted, a lesson is read to us on the danger 

 of relying too much on minute and indescribable differences in the 

 microscopic structure of a rock. 



b. The Nature of the " Granite^ — I have used the term granite 

 throughout, partly because it is so coloured on the Survey map, and 

 partly because, if one term only is to be chosen, this is the most gene- 

 rally applicable. But in reality under this one term a great variety 

 of rocks, not to mention those which are obviously distinct, must be 

 included, though I believe them aU to be connected portions of one 

 great mass. Perhaps the most typical granite is that found at 

 Hen-bias, near Llandrygarn, which on comparison with that from 

 Kingstown, near Dublin, shows a resemblance amounting to almost 

 absolute identity, the only differences being the less freshness of the 

 mica and the smaller individuals of the quartz. One is almost 

 tempted to ask if this can belong to a distinct and later eruption, 

 only it is easy to recognize that some at least of the others, as that 

 at Gualchmai, only differ by brecciation and alteration. The next 

 in order of alteration, after this at Hen -bias, is in a mass or tongue 

 between two diorites about a mile and a half to the north ; and the 

 next is the great mass seen in the cutting at Llanfaelog, where some 

 of the felspars are almost porphyritic, and all the mica has either 

 induced, or been developed in, the cracks. The granite near Craig- 

 yr-allor is tolerably whole, but the other samples that I have exa- 

 mined are all more or less broken, and become in some places regular 

 endoclastic breccias (shall we say " endoclasts " ?). They are, how- 

 ever, mostly from places rather near the boundary. Such is the 

 rock at Gualchmai, at Yr-ynys-goed, Coedana, and from the Llaner- 

 chymedd railway, near the ninth milestone. But the most brecci- 

 ated of all that I have examined comes from the interior, at Bryn 

 twrog, south of the railway just mentioned. The granite in the 

 two tongues near Porth-ceryg-defaid and at Maen-gwyn differs in 

 another way. It is not so much brecciated, but has a larger supply 

 than usual of clean white mica, similar, in the latter case, to the 

 mica developed in the neighbouring pelite. 



All these granites differ less in their internal structure than 

 would be expected from their external aspect. This latter is partly 

 dependent on the size of the crystals, and partly on the brecciation, 

 which has brought about the introduction of substances which alter 

 the appearance, but leave the intervening granite untouched. We 

 may take them all, therefore, to be parts of a single massif. In 

 two places, however, we find felsitic rocks, and, probably, there are 

 several more, as the granite has not been examined in every spot, 

 and these may be said to have been noted by accident. They are 



Q.J.G.S. No. 175. 2l 



