SoLLAS — On the StrucUire and Origin of Quartdte Rocks. 181 



grains, either by a sericite border or by tbe dotting of the in- 

 eluded area, we are enabled to explain the irregular mosaics 

 into which parts of the slice at first seem to be resolved, for within 

 one of the constituent areas of such a mosaic we can sometimes 

 discern a rounded grain as a nucleus over which a subsequent 

 growth of silica, distinguished by being free from included 

 cavities, has proceeded, till the interspaces between the grains 

 have been filled up. The percolation of waters containing silica in 

 solution may thus be admitted, but this is nothing more than has 

 already been shown by Bonney and Phillips to have occurred in 

 the case of many much younger sandstones and grits. 



But the alteration of the rock has not by any means been 

 altogether accomplished by the exclusive agency of percolating 

 waters : pressure has played a considerable part. Its effects are 

 recognised at once in the undulose and banded extinction of the 

 quartz grains, and in the cracks which bound the areas of different 

 optical orientation coexisting in the same grain. Its action in 

 consolidating the rock has been various, sometimes it has broken a 

 grain across, and forced the parts asunder, the gaps being after- 

 wards healed with a deposition of silica ; sometimes it has crushed 

 the grains, but the effects of crushing appear to be restricted to 

 certain lines, and of general marginal crushing there appears to 

 be scant evidence. The most general result of the pressure has 

 been, I believe, a slow yielding of the compressed grains, a viscous 

 flow having taken place by which the interstices between the 

 grains have become entirely obliterated. This is suggested by the 

 curious manner in which the small end of an egg-shaped grain 

 may be seen projecting into the broad side of a larger, rounder 

 one, against which it has been forced, thus repeating in 

 miniature precisely the same kind of adjustment which must 

 be familiar to everyone who has carefully studied the relations 

 of the pebbles in the pebble-beds of the Trias. It is further 

 borne out by the curious, often fan-shaped, fields of extinction 

 which directly arise from the region where tlie one grain intrudes 

 into the other, an effect of pressure which could not have arisen 

 had the two grains been always from the first accurately fitted 

 together, as they would have been had they been deposited in 

 place from solution. 



A further argument for the clastic origin of the rock lies in 



Q2 



