HISTORY OF GRITS AXD SAXDSTOXES. 



19 



Needle-like crystals of schorl are sometimes enclosed in this 

 quartz. 



The most considerable bed of sand at Bovey Heathfield, Devon- 

 shire, no. 27 of Mr. Pengelly* (Miocene?), consists largely of 

 quartzose fragments, nearly all of which are sharply angular, trans- 

 parent, and colourless. They contain fluid-cavities with bubbles ; 

 but the latter appear to be less numerous than in the quartz of some 

 Cornish granites. Schorl is present in considerable quantity, both 

 as detached crystals and' as portions of crystals, also as needles 

 penetrating quartz. 



Post-Tertiary.- — Sand washed from the Lower Boulder-clay at Holy- 

 well, Flintshire, is largely composed of small quartz pebbles, rounded 

 grains of various felspathic and other rocks, and numerous fragments 

 of millet-seed sandstone. A few unworn quartz crystals resulting 

 from the disintegration of crystalline sandstones, and some angular 

 grains of quartz, were also observed. Even the smallest particles of 

 this sand are often rounded. 



The larger grains of a sand of Middle Glacial age which occurs in 

 this locality are either rounded grains of quartz or of some other 

 rock, or small pebbles of millet-seed sandstone. Those of medium 

 size are millet-seed quartz grains, mixed with a few unworn crystals 

 and angular pieces of the same mineral. 



A specimen of Middle Glacial sand from Bagilt in the same 

 county differs in no respect from the foregoing, excepting that 

 crystals of quartz derived from crystalline sandstones are rather 

 more numerous, angular fragments are less rare, and broken millet- 

 seed grains are of more frequent occurrence. 



The Middle Glacial drift at Colwyn Bay is mainly composed of 

 small pebbles of various rocks, principally of quartz, with a few 

 unworn crystals of the same mineral, resulting from the disintegra- 

 tion of sandstones. In this drift the smallest fragments, although 

 generally rounded, have not been converted into minute pebbles. 



At different times I have examined numerous specimens of recent 

 water-borne sands. Among these, that on the sea-shore at Pentewan 

 int Cornwall is, as described, p. 24, perfectly sharp and angular, as 

 is the sand on the beach at Par, about six miles further east. Accord- 

 ing to Dr. Sorb} T such is also the case with regard to the sands of 

 the modern beach at Scarborough, and those of the river-terraces at 

 Dunkeld. 



A large proportion of the quartz in the sands of the Thames 

 valley is sharply angular, although mixed with rounded grains of 

 the same size. The grains of the auriferous sands collected on the 

 coast of Northern California are likewise for the most part angular, 

 although perfectly rounded ones are at the same time present. 



Among the blown or seolian sands which have been examined is 

 one from the Great African Desert, and another from Arabia 

 Petrsea. The grains of these are, without exception, much worn ; 

 and there is no admixture of the angular fragments found in 



* "The Lignites and Clays of Bovey Tracey," Phil. Trans. 1862, toI. clii. 

 p. 1019. 



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