I 



MACKIE — ON THE BOTTOM-ROCKS. 155 



causes ; so that this, as well as every other such isothermal line, would 

 in any given vertical section present the form of an irregular undu- 

 lating curve. 



Now wherever the deep oceans reposed, their water-masses would form 

 natural conductors of the internal heat, and beneath them the lines 

 of equal heat would recede more or less towards the centre. But 

 wherever great quantities of sediment were deposited, there the con- 

 ducting power of the ocean-water would be prevented from action, 

 and the isothermal lines below would ascend. Thus such deposits of 

 sand and mud would be e^ posed to the force of the subterranean heat ; 

 and these new strata, if of limestone or other similarly heat-affected 

 substances, would expand ; and the result would be an elevation of 

 territory. Thus, in North America, the great uplifted mass of the 

 Alleghanies (Apalachian Mountains) consists of Paleozoic sediments ; 

 or, in other words, that uplift or elevation was of post- Carboniferous 

 date. The Pyrenees, again, are of post-Cretaceous elevation ; the 

 Andes of South America, the Alps in Europe, and the Himalayas of 

 India are of post-Tertiary, or, more accurately, of post-Eocene date. 



If, on the other hand, the accumulated sediments subjected to the 

 action of the internal heat by the subterranean rise of the isothermal 

 lines were of aluminous or other similarly heat- affected mineral mate- 

 rial, a contraction might take place ; and, instead of an uplift, an 

 extended depression, deepening the abyss of the ocean, might result ; 

 and thus, by the various combinations, oppositions, and modifications 

 of these expansive or contractile operations, new lines, or double, or 

 parallel lines of elevation might be formed ; or the original lines of 

 uplift and consequent weakness may either have been extended, like 

 the successive extensions of the cracks of a starred pane of glass, by 

 every thermal variation, or have been altogether broken down. 



By this rise of the range of the internal calorific influence up to, and 

 its action upon, the inferior portions of the accumulated sediments, 

 various kinds of granitoid and gneissic rock would have originated ; 

 the granite being the lowermost portion fused, so to express it, under 

 intense pressure of the superincumbent heap, in the presence of water, 

 of a temperature perhaps equal to red-heat. As this granitized mass 

 was forced up by its own expansion, it fissured the semi-crystalline 

 and unchanged strata above it, dragging up, like a giant on its shoulders, 



M 2 



