528 :rey. j. r. blake o^' the 



are such as require fresh ingredients. "We cannot, therefore, call 

 the latter more metamorphosed. We may, however, look upon the 

 entirely crystalline portions as having taken up their character under 

 the influence of heat, and so rendered themselves less alterable 

 by the aqueous agents of metamorphism, which would act upon the 

 less crystalline portions in a different manner, and so introduce 

 fresh minerals, and bring with them, if necessary, the fresh in- 

 gredients required. But what can be that form of heat that 

 shall thus act so sporadically? In considering this, we must 

 not forget the stratigraphy, nor overlook the minor indications 

 which special forms of rocks may afford. jN'ow, besides the 

 hornblendic rocks further west by the side of Parys jVTountain, 

 there is, near Porth-y-gwichiad, a mass of rather fresh holo- 

 crj^stalline rock which is composed of calcite, enstatite, and 

 augite, apparently belonging to the series, and not a later dyke ; 

 and not far from the same spot there is also a mass of fine felsitic 

 dust, with parallel flakes of sericite, just like a rock we find at 

 Pant-yr-Eglwys ; and these so-called granites themselves contain 

 more acid and more basic varieties. How can we account for all 

 these by the agency of heat upon the same set of rocks ? But all of 

 them are of such materials as are found in volcanic regions, and the 

 whole would be accounted for if the holocrystalline portions repre- 

 sented the lavas, and the other parts the ashes of an eruptive area. 

 The question then arises, Are the products of a modern volcano of 

 similar size inextricably mixed in the same way as these are, and 

 is it as difficult there to separate one part which may be called a 

 lava from one which is an ash ? To solve this question I visited 

 last spring the volcanos of Southern Italy. Here, in the Phlegraean 

 fields and in the Island of Ischia, I found the crystalline and 

 fragmentary portions and the eruptions, when occurring together, to 

 be in the same confusion. The trachyte has no clear boundary 

 from the trachyte-tuff. The finer materials become stratified, and 

 in the more compact and less stratified portions irregular hard lumps 

 occur composed of similar but more crystalline materials. Still closer 

 resemblances are seen in the miniature volcanos of the Lipari 

 Islands, where the great variety of rocks which unite to form the 

 cones and surrounding heaps in Lipari itself and in Panaria, though 

 occasionally separable enough into lava-flows and ashes towards the 

 centres of eruption, are inextricably mixed up at their margins, 

 and the same confusion arises. The geologist in Anglesey is puzzled 

 to account for the curious association of rocks of different structure, 

 yet of similar materials, that he there finds brought together, 

 and the geologist among the volcanos of the Mediterranean is 

 equally puzzled to explain how similar materials are likewise 

 arranged ; but the problem is the same in both cases, and we may 

 safely here apply the principle, though it be not of universal 

 application, that similar results arise from similar causes. This, at 

 least, is the most probable solution. In other words, the rocks 

 near Llanenllwyfo are not of ordinar}- sedimentary origin, but are 

 the products of a small and, for the most part, ash-producing 



