Ch. VIL] 



ALLUVIUM. 



81 



of these plains differed from the first or upland alluvium, by containing 

 in it rounded fragments of various volcanic rocks, and often bones be- 

 longing to distinct groups of land animals which flourished in the country 

 in succession. 



The annexed drawing will explain the different heights at which beds of 

 lava and gravel, each distinct from the other in composition and age, are 

 observed, some on the flat tops of hills, TOO or 800 feet high, others on 

 the slope of the same hills, and the newest of all in the channel of the 

 existing river where there is usually gravel alone, but in some cases a nar- 

 row stripe of solid lava sharing the bottom of the valley with the river. 

 In all these accumulations of transported matter of different ages, the bones 

 of extinct mammalia have been found belonging to assemblages of land 

 quadrupeds which flourished in the country in succession, and which 

 vary specifically, the one set from the other, in a greater or less degree, 

 in proportion as the time which separated their entombment has been 

 more or less protracted. The streams in the same district are still under- 

 mining their banks and grinding down into pebbles or sand, columns 

 of basalt and fragments of granite and gneiss ; but portions of the 

 older alluviums, Avith the fossil remains belonging to them, are prevented 

 from being mingled with the gravel of recent date by the cappings of 

 lava before mentioned. But for the accidental interference, therefore, of 

 this peculiar cause, all the alluviums might have passed so insensibly the 

 one into the other, that those formed at the remotest era might have 

 appeared of the same date as the newest, and the whole formation might 

 have been regarded by some geologists as the result of one sudden and 

 violent catastrophe. 



In almost every country, the alluvium consists in its upper part of 

 transported materials, but it often passes downwards into a mass of 

 broken and angular fragments derived from the subjacent rock. To this 

 mass the provincial name of " rubble," or " brash," is given in many 

 parts of England. It may be referred to the weathering or disintegra- 

 tion of stone on the spot, the effects of air and water, sun and frost, and 

 chemical decomposition. 



The inferior surface of alluvial deposits is often very irregular, con- 

 forming to all the inequalities of the fundamental rocks (fig. 100). Oc- 

 casionally, a small mass, as at c, appears 

 detached, and as if included in the subja- 

 cent formation. Such isolated portions are 

 usually sections of winding subterranean 

 hollows filled up with alluvium. They 

 may have been the courses of springs or 

 subterranean streamlets, which have flowed 

 through and enlarged natural rents ; or, 

 when on a small scale and in soft strata, 

 they maybe spaces which the roots of large 

 trees have once occupied, gi-avel and sand 

 having been introduced after their decay. 



Fig. 100. 



a. Vegetable soil. &. Alluvium. 



c. Mass of same, apparently detached. 



