Theoretical Considerations. 351 



if we suppose the source of heat to be in the interior of the earth, it 

 must have operated more or less on every layer in the earth's crust : 

 and, therefore, if this theory be true, probably all the older rocks have 

 been more or less modified by heat. That the crust of the globe has 

 been subject to the action of powerful and long continued heat, it 

 seems to me no one acquainted with geology in its present state can 

 doubt. 



According to the theory just described, (I have not space fully to 

 defend and illustrate it, but Dr. Macculloch has done it in his System 

 of Geology,) argillaceous slate is nothing but shale, which has been 

 subject to heat, and perhaps increased pressure : quartz rock may have 

 resulted entirely from aqueous deposition ; or it may have been subse- 

 quently indurated by heat. Between mica slate and micaceous sand- 

 stone, there is a most striking analogy: indeed, it is no easy matter to 

 distinguish by the eye between the specimen of sandstone No. 177. 

 from Northampton, near a mass of trap, and several specimens of mica 

 slate. The same is true of the micaceous sandstone near Turner's 

 falls, which are also in the vicinity of trap. The supposed origin of 

 the other stratified primary rocks, I shall point out in treating of 

 them. 



As to the elevation and dislocation of the strata of rocks, particular- 

 ly the primary, I shall say more in another place. But the manner 

 in which the numerous flexures and contortions which their laminae 

 present, were produced, deserves consideration in this connec- 

 tion ; for these are more common in our mica slate than in any other 

 rock ; not even excepting gneiss. 



These curvatures may have been produced before or subsequent to 

 the consolidation of the rock. I have already pointed out the striking 

 resemblance between the slightly undulating surface of some of the 

 shales in the new red sandstone, and the gentle ridges and depressions 

 at the bottom of almost every large river arid pond, or on the margin of 

 the ocean. And if argillaceous and mica slate had that origin which 

 is supposed by the preceding theory, the analogous irregularities, so 

 frequent on the surface of argillaceous and mica slate, may be referred 

 to the same cause as that which produced them on the shale. 



By comparing the sections which I have given of flexures in the 

 newest tertiary formation, with those in our mica slate, it will be seen 

 that the latter might have had their origin in the former, if we admit 

 the theory that has been advanced of the origin of mica slate. It maybe 

 difficult, indeed, to account for these flexures in the clay : but that they 



