94 ATTRITION OF GRAINS IN SANDSTONES. 



quartz deposited upon them from water infiltrating through the rock 

 so as to present the aspect of perfect crystals absolutely free from wear. 

 This deposited quartz appears to be almost always derived from the 

 decomposition of associated felspar in the sand, though in the Yosges 

 district M. Daubree finds reason for attributing it to the local action of 

 heated water ; but since the china clay always develops from its free 

 silica sharp crystals which sometimes reach a length of three inches, we 

 have in this action a sufficient explanation for the crystalline condition 

 observed in so many British sandstones. By placing a number of 

 grains of sand from a given deposit under the microscope, and count- 

 ing the proportion of worn to unworn grains, Dr. Sorby has shown 

 that the sand in several formations is derived from different sources. 

 He observes that the sand from the Boulder Clay at Scarborough is 

 fresh and angular, showing few or no rounded grains, and that the 

 modern beach at Scarborough, which is largely derived from the 

 Boulder Clay, has the grains scarcely more worn. In the Thanet Sands 



Fig. 41. Grains of sand magnified, with quartz crystals upon and around the grains. 

 Alter J. A. Phillips and Sorby. 



at Crossness, and in the Hastings Sands at Hastings, one-half of the 

 grains are worn. In the Upper Greensand the amount of wear in- 

 creases as we pass from Dartmoor to the east, one-tenth of the grains 

 being worn at Haldon Hill, one-fifth in the Isle of Wight, one-third in 

 Sussex, and one-half in Kent, indicating that as the deposit recedes 

 from the region from which it was derived the amount of wear 

 increases. Mr. J. A. Phillips, F.R.S., has examined the chief British 

 sandstones, and it may be useful here to give a brief summary of his 

 conclusions with regard to some of the more important. 



The Barmouth grits in the Lower Cambrian series of North Wales, 

 between Barmouth and Harlech, often enclose angular fragments of 

 quartz fully a quarter of an inch in diameter, ^he deposit is an aggre- 

 gation of quartz and felspar united by a silicious cement tinged greyish 

 green with a mineral which is probably chlorite. The quartz contains 

 a few fluid cavities with moving bubbles. The felspar is of two kinds, 

 orthoclase and probably oligoclase. Some of the quartz contains 



