70 SECONDARY SERIES. 
that the great corn fields, and the greatest popu- 
lation of the world, are placed on strata of the 
secondary and tertiary formations ; or on their 
detritus, composing still more compound, and 
consequently more fertile diluvial, and alluvial 
deposits.* 
Another advantage in the disposition of stra- 
tified rocks consists in the fact that strata of 
limestone, sand, and sandstone which readily 
absorb water, alternate with beds of clay, or 
marl, which are impermeable to this most impor- 
tant fluid. All permeable strata receive rain- 
water at their surface, whence it descends until 
it is arrested by an impermeable subjacent bed 
of clay, causing it to accumulate throughout the 
lower region of each porous stratum, and to form 
extensive reservoirs, the overflowings of which on 
the sides of valleys constitute the ordinary 
supply of springs and rivers. These reservoirs 
are not only occasional crevices and caverns, 
but the entire space of all the small interstices 
* It is no small proof of design in the arrangement of the ma- 
terials that compose the surface of our earth, that whereas the 
primitive and granitic rocks are least calculated to afford a fertile 
soil, they are for the most part made to constitute the mountain 
districts of the world, which, from their elevation and irregulari- 
ties, would otherwise be but ill adapted for human habitation ; 
while the lower and more temperate regions are usually composed 
of derivative, or secondary strata, in which the compound nature 
of their ingredients qualifies them to be of the greatest utility to 
mankind, by their subserviency to the purposes of luxuriant vege- 
tation. — Buckland's Inaugural Lecture, Oxford, 1820, p. 17. 
