ANNIVEESAKY ADDKESS OF THE PEESIDENT. 75 



green glass. On the whole it would be impossible to meet with a 

 better illustration of an ash due to a more or less glassy felspathic 

 lava. Quartz appears to be absent from this particular specimen, 

 except as filling up a few cavities ; and unless it be in the form of 

 minute needles, met with here and there in the glassy basis, augite 

 does not occur. To this circumstance may be attributed the very 

 small amount of green material subsequently formed in the rock. 

 In some other specimens grains of quartz do occur ; and it is in- 

 teresting to find that the cavities sometimes contain no liquid, 

 which probably has been driven off by heat, in the same manner as 

 from grains of sand in the stratified deposits in contact with the 

 erupted rocks of Salisbury Crags, described further on. In other 

 localities the ash must have contained a much more perfect pumice, 

 and in others a considerable amount of augite, subsequently altered 

 into a green fibrous mineral, which also entirely or partially fills 

 what were originally cavities in the rock. Sometimes, even in the 

 same microscopical section we can trace a gradual passage from 

 ash composed of particles large enough to be easily recognized, to 

 what appears to have been an extremely fine-grained volcanic dust, 

 associated with fragments of true pumice. At Ambleside some 

 comparatively large fragments of pumice occur, with their cavities 

 filled with calcite, in an otherwise fine-grained slate, in the same 

 manner as they so often occur in fine-grained oceanic mud. This, 

 of course, is easily explained, since their unusual buoyancy would 

 enable them to float where denser fragments could not be drifted. 



2. Rocks more or less considerably altered. — Passing from cases 

 where the original nature of the material is sufficiently well shown, 

 we come by degrees to others in which it has been more or less 

 changed since deposition, until we arrive at cases in which it is 

 difficult or impossible to decide from the structure alone whether 

 the rock was erupted and subsequently changed by water, or was 

 an ash of similar material afterwards changed by water and heat. 

 There appears to be no doubt that certain minerals have been 

 formed in situ, such as epidote and the various green substances 

 described further on, and also no doubt that the augite, much of 

 the felspar, and the garnets were original constituents of the ash ; 

 but it is sometimes difficult or impossible to say whether the 

 felspar microliths were or were not formed in situ. On the whole 

 it is most probable that those met with in some specimens were 

 deposited as ash, whilst those in others were due to subsequent 

 alteration. 



The first stage in the alteration was probably the filling-up of 

 cavities and the spaces between the particles with calcite, quartz, 

 and hydrous silicates, so as to consolidate the rock. This change 

 may, I think, be referred to the decomposition of the augite and 

 other unstable materials of volcanic ash, and might not require any 

 materially elevated temperature. The formation of felspar and 

 epidote was doubtless greatly favoured by the very imperfectly 

 decomposed condition of the original material, but probably re- 

 quired a more elevated temperature. 



