the Earth's Contraction from Cooling. 283 



with very little potash or soda ; and in these constituents there 

 is comparatively little variation. 



Hunt claims that the presence of moisture in many igneous 

 rocks sustains the idea of derivation from the fusion of sediments 

 — that is, from the fusion of rocks containing water. But it 

 has been shown (on page 280) that the cases of hydrated dole- 

 rite in the region referred to are local and exceptional, and that 

 the ridges are so mixed up with those of the more common an- 

 hydrous dolerite that all must have had one source. His argu- 

 ment would be of some weight if there were no source for the 

 moisture except in the region of fusion. The hydrated rock, 

 moreover, is shown to have the low percentage of silica which 

 belongs to the anhydrous dolerite. 



If subterranean fusion were somehow produced and a zone of 

 mobile rock were thus made many scores of miles deep, with the 

 material not simply plastic but of perfect liquidity, and the zone 

 without obstructions in any part, so that there could be tidal 

 movements or at least free-flowing currents throughout it, then, 

 if the material entered into fusion were as diverse as the rocks 

 of the Atlantic border, there might be such a mixing up and 

 chemical digestion of limestone and every thing else, that the 

 product would perhaps be doleritic — a rock low in silica (or 

 basic). But these conditions are those of the earth's liquid en- 

 velope before and after refrigeration began, and of the viscous 

 layer developed out of it, even probably to the basic character 

 of the fused material ; and further, they are what Hunt's method 

 has no provision whatever for producing. 



The facts afforded by other parts of the globe and other pe- 

 riods in its history add force to this argument. A doleritic or 

 basaltic character is the prevailing one among the igneous erup- 

 tions of all ages from Archaean time to the present, and of all 

 continents and oceans. Whitney and Pumpelly have described 

 basic or doleritic rocks of Lower Silurian age from the Lake- 

 Superior region, where they are the prevailing igneous rocks. 

 Hunt has described and analyzed others, from Canadian locali- 

 ties, of the Laurentian, the Lower Silurian era, and of later 

 Palaeozoic time. They are far the most abundant of the igneous 

 products of the Tertiary and Quaternary over the Pacific slope. 

 Similar facts might be cited from other continents*. More 

 than three fourths of true igneous rocks are probably of this 

 nature. Hence it is reasonable to pronounce such ejections 



* For comparison with the analyses on page 279, I add here others of 

 the dolerite (trap) of (I.) the Giant's Causeway, (II.) Staffa, and (III.) Faroe, 

 by Streng (Pogg. Ann. 1853, vol. xc. pp. 110, 114 — all of which are now 

 supposed to be of Tertiary origin, — and another (IV.) of an Archaean (Lau- 



