322 A. OEIKIE ON THE SUPPOSED 



shows a tendency towards the aggregation of granular crystalline 

 quartz into nests and fine threads traversing the base, as if the 

 quartz porphyry had been in some measure affected by the same 

 process which has resulted in the production of the siliceous aggre- 

 gations. Mr. Davies has also pointed out that in the felsite of Eoche 

 Castle abundant fissures filled with crystalline quartz occur, together 

 with bands of fibrons chalcedony *. 



Prom the marked development of the siliceous microcrystalline 

 aggregations near the dykes and bosses of porphyry, and their 

 paucity or absence elsewhere, I infer that they are connected with 

 the protrusion of that rock. That water was present, probably in 

 considerable quantity, in the magma out of which the porphyries 

 solidified, is shown by the liquid inclusions in the quartz of these 

 rocks. Hot water or aqueous vapour no doubt contin*ued to escape 

 from the eruptive masses long after they had become solid. This 

 water, probably sometimes charged with alkaline materials, sometimes 

 with dissolved silica, would most readily escape along the highly 

 inclined planes of stratification. Moreover the interstitial water 

 present in the beds, or which might reach them from the surface, 

 would come within the influence of the heat of the eruptive rock. 



That the changes which produced the present microcrystalline 

 texture and diffusion of quartz in these rocks took place in the 

 presence of water, and under considerable pressure, is proved by the 

 important fact to which I have referred, that the qnartz of the fine 

 veins that traverse the adinole is crowded with liquid inclusions with 

 freely moving bubbles. 



The presence of water being thus established, it is obvious that 

 the nature of the metamorphisn which it might set up would depend 

 not only on its own chemical activity and that of the substances 

 dissolved in it, but also upon the chemical composition of the rocks 

 affected. Some of the strata must undoubtedly have been more 

 liable to undergo alteration than others. Probably those which have 

 suffered most, consisted originally of finely divided felsite dust. 

 Such a condition would offer peculiar facilities for alteration by water 

 under pressure. The presence of qnartz-grains here and there in 

 the sediment might determine the deposit of additional silica round 

 these centres. The water permeating the rock would, no doubt, 

 also fill the fine fissures with the same material. A crystalline re- 

 arrangement of the felspathic constituents was likewise set on foot, 

 with the result of producing a finely crystalline mass through which 

 larger crystals, especially of plagioclase, have been developed. Prom 

 the marked proportion of soda in the analysis of the adinole from 

 Nun's Chapel, some of the felspar thus developed is obviously 

 albite. We may suppose that the alkali was, in great measure, 

 supplied during the process of metamorphism. The analysis 

 further shows that there must be free silica as well as more alumina 

 than is required for the constitution of the felspar. 



The subject is too wide for adequate discussion here. But, from 

 the data which I have now brought forward, it will be evident that 

 * Quart. Journ. Geol. Soc. vol. xxxv. p. 291. 



