FORMS OF SAND GRAINS. 93 



evidence that the quartz was derived from a rock which solidified from 

 a heated condition under great pressure. The fluid cavities are most 

 numerous in the Cornish granites, and there they often contain cubic 

 crystals of alkaline chlorides. The schists of Scotland contain but 

 few fluid cavities, and crystals have never been observed in the cavi- 

 ties of these rocks, or in any Scotch granite. There is a further differ- 

 ence between the Scotch and Cornish granites in the fact, that the 

 former generally contain in the grains of quartz fine hair-like crystals 

 of the mineral rutile, while in the latter the grains abound in small 

 prisms of tourmaline. The quartz crystals of some volcanic rocks, 

 like the rhyolites, sometimes contain six-sided or imperfectly rhombic 

 enclosures of coloured glass with accompanying bubbles. The presence 

 of volcanic glass would always indicate denudation of a volcanic rock. 

 The grains of sand are rarely obtained direct from the rock which yields 

 them without experiencing a large amount of wear. This attrition is 

 due to transport of the material by rivers, and grinding by the waves 

 on the sea-shore. Some ancient sand-beds are made up of grains which 

 are unworn and practically new, while the grains on many a modern 

 sea beach are of vast antiquity, and have formed part of several 

 geological formations, in each of which they have been worn. When 

 we examine some of the modern sands in process of formation, 

 the amount of wear is found to be unexpectedly small ; thus the sand 

 of the river-terraces at Dunkeld is almost entirely angular, and pre- 

 sents the features characteristic of sand derived from schists. The 

 sands of the Arabian, Egyptian, and great African deserts, on the other 

 hand, are exceptionally worn, every grain presenting the characters of 

 a miniature pebble, a feature which results from the agency of wind 

 in triturating the grains against each other. The sand which occurs 

 in the decomposed granite of Cornwall is separated from the rock by 

 artificial washing in the china clay works of that district, and the 

 characters of the grains have been observed in the St. Austell river, 

 which has a fall of from 150 to 470 feet to the sea. Here the 

 grains are seen to consist of a mixture of quartz, felspar, and a little 

 mica. They vary in size from over ^th inch to a fine sand of less 

 than T -g-oth inch. 



The fragments of quartz and tourmaline are sharp and unrounded, 

 but the edges of the mica and felspar are distinctly rounded. The 

 felspar grains always have an external coating of clay due to decom- 

 position. As the sand becomes finer the grains become less worn, and 

 the proportion of mica is larger than in the coarse sand, and there is 

 less felspar. Attempts have been made by Professor Daubre"e to esti- 

 mate the distance that a grain must travel to show a definite amount 

 of wear, and he concludes that before a grain of quartz -g^th inch in 

 diameter can assume the appearance of a miniature pebble it must 

 undergo the same amount of abrasion as would result from travelling 

 three thousand miles along a shore. Grains, however, from being 

 corroded in the manner already indicated, sometimes become modified 

 in outline without much wear, and occasionally they are singularly 

 fractured ; but frequently grains which were rounded by attrition have 



