ANNIVERSAET ADDRESS OP THE PEESIDENT. 7 1 



base often contain as perfect empty, more or less spherical, bubbles 

 as bad artificial glass. The quartz grains also appear to have been 

 derived from a quartz-felsite, since they not only have the charac- 

 teristic crystalline shape and internal structure of such quartz, but 

 occasionally are attached to or enclose portions of the felsitic base. 

 The structure of this base varies considerably in different fragments, 

 but corresponds closely with that of the various massive felsites found 

 in the district. Sometimes these are simply felspathic, but usually 

 contain a large amount of a micaceous mineral, easily recognized by 

 its shape and optical characters. The amount of granules and 

 needles of magnetite or ilmenite varies much, even in different parts 

 of the same fragment. Occasionally we see black grains of basalt, 

 and sometimes fragments of what may have been augite altered into 

 a deep-green laminar mineral, which we may call chlorite, quite un- 

 like the mica in the felsites. Judging from analogy, the associated 

 augite or olivine was probably the source of the more considerable 

 amount of the same green chloritic mineral which fills the cavities 

 in the imperfect pumice and the interspaces between the constituent 

 fragments of the w^hole rock. It has also to a slight extent pene- 

 trated into the felsitic fragments, and appears to have partially re- 

 placed the felspar ; so that some fragments may be looked upon as 

 imperfect pseudomorphs. A few grains may have been derived 

 from an older slate ; but yet, on the whole, I think w^e ought to 

 regard the rock as a consolidated quartzose felsitic ash. 



If the micaceous basis of this ash were worn down in the crater 

 into dust, or more completely decomposed by weathering, and the 

 material afterwards sorted by gentle currents of water, it would 

 yield a deposit corresponding in all essential particulars with the 

 fine-grained slates of Penrhyn and Llanberis, allowance, of course, 

 being made for subsequent chemical and mechanical changes. Thus, 

 for example, several different laminar chloritic or talcose minerals 

 have crystallized out, not, as in the coarse-grained rock, only in the 

 cavities, but have pushed aside and displaced the surrounding ma- 

 terial. Vast numbers of small red crystals of specular iron or very 

 minute black needles of some other less-oxidized iron mineral have 

 been formed in situ ; whilst occasionally calcite and quartz have 

 been deposited in crystals of some size or disseminated through 

 ^ the rock, so as to greatly increase its solidity. The structure has 

 also been much altered by the pressure which gave rise to cleavage. 



Other beds in the same group of rocks differ very widely from 

 those just described, and contain little mica and much material 

 analogous to kaolin, as though either derived from the decom- 

 position of ash or rocks containing much less mica and more 

 felspar than that which yielded the material of the Penrhyn and 

 Llanberis slates, or separated from the more micaceous material by 

 currents. 



It is perhaps scarcely needful to say that when I speak of the 

 material having originated from any particular kind of rock, I mean 

 that it did so in the first instance. It by no means follows that it 

 was so derived directly, and was not previously deposited and again 



