HISTORY OF GRITS AND SANDSTONES. 



25 



angles and edges, the operation will gradually become slower as the 

 surfaces become more worn and the weight of the grain decreases. 



That angular fragments of quartz having a diameter of less than 

 -gij inch remain unrounded by the continuous action of breakers after 

 many years' exposure, is evident from an examination of the sands 

 at Pentewan. It has been shown by other evidence, as well as by 

 the recent experiments of Professor Daubree, that the rounding-down 

 of such sands by the action of running water must be an exceed- 

 ingly slow operation, and one requiring a somewhat active current 

 with an amount of friction equivalent to transport over enormous 

 distances. Grains of quartz of similar dimensions are, in blown 

 sands, completely rounded. 



Summary and General Conclusions. 

 » 



The Cambrian grits of Barmouth contain quartz and felspar, both 

 in the form of angular fragments and also as rounded pebbles. The 

 materials presenting these different forms have probably been de- 

 rived from two distinct sources ; while the large size and complete 

 sharpness of the angles of many of the irregular grains appear to 

 indicate that they cannot have been transported from any consider- 

 able distance, and that the felspar cannot have been derived from 

 kaolinized granite. 



All the arenaceous rocks of Silurian age which have been ex- 

 amined contain a small proportion of felspar, the grains of the 

 various constituent minerals being in some cases angular, and in 

 others rounded. 



Many of the rocks belonging to this period are composed of a 

 mixture of grains of both forms. Among rocks mainly composed of 

 rounded grains are the Stiper Stones of Shropshire and the Lower 

 Lickey Quartzites of Westmoreland. Some of the grits from the 

 neighbourhood of Aberystwith enclose fragments of a volcanic rock 

 of doleritic character. 



The grits of Cornwall, which are of at least Devonian age, include 

 flakes of soft slaty rocks, the edges of which are perfectly sharp, 

 together with angular fragments of the well-known " greenstones " 

 and " dunstones" of that county. 



A large number of the Carboniferous, Permian, and Triassic 

 sandstones are composed almost entirely of quartz crystals, which 

 have undoubtedly been produced in situ, as they not only penetrate 

 and interpenetrate one another, but also exhibit the most perfect 

 sharpness and freshness of outline. As confirmatory of this hypo- 

 thesis it may be mentioned that in a quarry at Eoggen Tor, on 

 Dartmoor, the felspar has in places become decomposed into soft 

 kaolin, in which the liberated silica is imbedded in the form of 

 aggregations of well-formed and transparent quartz crystals *. 

 Unworn double-pointed crystals of quartz, likewise resulting from 

 the decomposition of felspar, have recently been found near St. 

 Austell, Cornwall, in soft china-clay ; one of these, more than three 



* Quart. Journ. Geol. Soc. vol. xxxvi. p. 9. 



