56 PRIMARY STRATIFIED ROCKS. 
incandescent elements, wholly incompatible with 
any condition of life, which can be shown to 
have ever existed, formed the entire substance of 
the globe.* 
* In adopting the hypothesis that the primary stratified rocks 
have been altered and indurated by subjacent heat, it should be 
understood, that although heat is in this case referred to as one 
cause of the consolidation of strata, there are other causes which 
have operated largely to consolidate the secondary and tertiary 
strata, which are placed at a distance above rocks of igneous 
origin. Although many kinds of limestone may have been in 
certain cases converted to crystalline marble, by the action of heat 
under high pressure, there is no need for appealing to such 
agency to explain the consolidation of ordinary strata of carbonate 
of lime ; beds of secondary and tertiary sandstone have often a 
calcareous cement, which may have been precipitated from 
water, like the substance of stalactites and ordinary limestone. 
When their cement is siliceous, it may also have been supplied 
by some humid process, analogous to that by which the siliceous 
matter of chalcedony and of quartz is either suspended or dis- 
solved in nature ; a process, the existence of which we cannot 
deny, although it has yet baffled all the art of chemistry to imi- 
tate it. The beds of clay which alternate with limestone, and 
sand, or sandstone, in secondary and tertiary formations, show no 
indications of the action of heat; having undergone no greater 
consolidation than may be referred to pressure, or to the admix- 
ture of certain proportions of carbonate of lime, where the clay 
beds pass into marl and marlstone. Beds of soft unconsolidated 
clay, or of loose unconsohdated sand, are very rarely if ever found 
amongst any of the primary strata, or in the lower regions of the 
transition formation ; the effects of heat appear to have converted 
the earlier deposits of sand into compact quartz rock, and beds 
of clay into clay slate, or other forms of primary slate. The rock 
which some authors have called primary grauwacke, seems 
to be a mechanical deposit of coarse sandstone, in which the form 
of the fragments has not been so entirely obliterated by heat, as 
in the case of compact quartz rock. 
