450 SKETCH TOWARDS A 



rieties depend, partly on the original materials, and 

 partly on the times and modes of cooling. The ex- 

 ceptions are those where the materials have been fused 

 without effusion : in which cases, fragments of the 

 original strata sometimes remain. 



All the primary strata having originally been what 

 the secondary ones now are, the latter demand the 

 first place. Their materials are sand alone, or sand 

 mixed with clay, or clay alone ; or either, or both of 

 these together, mixed with calcareous earth, or with 

 hydrocarbonaceous matter, or with both, or lastly, car- 

 bonat of lirne. In these cases, they are strata of a fine 

 texture : but they are coarse or conglomerate, by con- 

 taining the fragments, instead of the materials, of pre- 

 vious rocks, as they may also contain the minerals of 

 those. The latter demand no further notice : the for- 

 mer can consist of nothing but sandstones, shales, and 

 limestones ; each, and the two former, especially, va- 

 rying in quality according to the ingredients. These 

 two are separate strata, in consequence of the differing 

 specific gravities of their materials in water, and be- 

 cause those have been the produce of former rocks. 

 This is also partially true of the later limestones at 

 least : but the great mass of these is the produce of 

 shells, breeding beneath the sea ; while, often pre- 

 serving those bodies, they are conchiferous limestones. 

 And the same reasoning applies to the tertiary strata. 

 I have attributed their consolidation to solutions of 

 silica and of carbonat of lime, or to pressure, or to 

 both united. But they have possibly, or probably, 

 been exposed to heat, as high up as the old red sand- 

 stone, if not higher, so as to have suffered changes of 

 texture : the arenaceous strata thus acquiring the as- 

 pect of quartz rock, and the shales that of primary 

 schist. 



