62 SECONDARY SERIES. 



and the farther addition of calarious earth produces a soil 

 highly valuable to the agriculturist : and where the natural 

 proportions are not adjusted in the most beneficial manner, 

 the facilities afforded by the frequent juxta-position of lime, 

 or marl, or gypsum, for the artificial improvement of those 

 soils which are defective in these ingredients, add materially 

 to the earth's capability of adaptation to the important of- 

 fice of producing food. Hence it happens that the great 

 corn-fields, and the greatest population 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 deposites.* 



Another advantage in the disposition of stratified rocks 

 consists in the fact that strata of limestone, sand, and sand- 

 stone 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 imper- 

 meable 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 

 of those lower parts of each permeable stratum, which are 

 beneath the level of the nearest flowing springs. Hence if 

 a well be sunk to the water-bearing level of any stratum, 



* It is no small proof of design in the arrangement of the materials that 

 compose tlie surface of our earth, tliat 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 irregularities, would otherwise be but ill adapted for human 

 iiabitation; while the lower and more temperate regions are usually com- 

 posed 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 vegetation. — Buckland's In- 

 augural Lecture, Oxford, 1820, p. 17. 



