PART I. CHAPTER VI. 



83 



Alluvium. 



fine from the coarse soil.* Immediately beneath this mould the 

 regular or fundamental stratified or unstratified rocks of the dis- 

 trict may appear; but there usually intervenes, if not an alluvial 

 mass, at least a quantity of broken and angular fragments of 

 the subjacent rock, provincially called rubble, or brash, in many 

 parts of England. This last may be referred partly to the wea- 

 thering or disintegration of stone on the spot, the effects of air 

 and water, sun and frost, and chemical decomposition, and partly 

 to the expanding force of the roots of trees, v/hich may have 

 grown in small crevices, at former geological periods, though 

 they may now be wanting. Sometimes the vibrations and undu- 

 lations of earthquakes may have had power, at some former era, 

 to shatter a surface previously rent and weathered. Thus in Ca- 

 labria, subterranean movements have been known to throw up 

 into the air the slabs of a stone pavement;! and Mr. Darwin 

 mentions, that in the Island of Quiriquina, in Chili, some narrow 

 ridges of hard primary slate, which is there the fundamental rock, 

 were as completely shivered by the vibrations of the great earth- 

 quake of February, 1835, as if they had been blasted by gun- 

 powder. The efiect was merely superficial, and had caused 

 fresh fractures and displacement of the soil, the slate below re- 

 maining solid and uninjured.:}: 



Alluvium differs from the rubble or brash, just described, as 

 being composed of sand and gravel, more or less rolled, in part 

 local, but often in great part formed of materials transported from 

 a distance. The term is derived from alluvio, an inundation, 

 or alluo, to wash. The gravel is rarely consolidated, often un- 

 stratified, like heaps of rubbish shot from a cart, but occasion- 

 ally divided into wavy and oblique layers, marking successive 

 deposition from water. Such alluvium is strewed alike over in- 

 clined and horizontal strata, and unstratified rocks ; is most abun- 

 dant in valleys, but also occurs in high platforms, and even on 

 lofty mountains, that of the higher grounds usually differing from 

 that found at lower levels. 



The inferior surface of an alluvial deposit is often very irregular, 

 conforming to all the inequalities of the subjacent rock. (Fig. 83.) 

 Occasionally a small mass, as at c, appears detached, and as if in- 

 cluded in the subjacent formation. Such isolated portions are usu- 

 ally sections of winding subterranean hollows filled up with allu- 

 vium. They may have been the courses of springs or subterra- 



* See Proceedings of Geol. See. No. 52. p. 574. Darwin on Formation of 

 Mould. 



t See Principles of Geology, Index, " Calabria." 



X Darwin, Journal of Travels in South America, &c., 1832 to 1836, in Voyage 

 of H. M. S. Beagle, p. 370. 



