286 Prof. J. D. Dana on some Results of 



feebler source of heat and fusion. Still, under the fractures 

 and shovings and crushings which must at times take place, it 

 should be sufficient ; and acting at infra- Archaean depths, it 

 would give uniform results over wide areas, or, on the other 

 hand, a degree of diversity. Unlike Hunt's hypothesis, Mallet 

 gives a reasonable source for the heat occasioning fusion. 



But the sufficiency of the method for all cases of igneous 

 ejections may well be questioned. The subsidence ending in 

 the fissures and trap-ejections of the Atlantic slope from Nova 

 Scotia to North Carolina was extremely slow, and probably 

 nowhere exceeded 5000 feet; and it caused in the end only 

 small displacements of the strata. In such a case as this the 

 motion would seem to be a wholly inadequate cause of the fusion 

 and ejections. Moreover, if the dependence of the subsidence 

 upon the existence beneath of a great region of mobile rock was 

 a fact, as has been urged*, there was fusion enough without aid 

 from this source. 



Further, over the Pacific slope of the Rocky Mountains the 

 vast ejections in the Tertiary era appear to have had, as I have 

 said, a natural source in an undercrust fire-sea, and the same 

 that was essential to all the previous oscillations of the crust, and 

 which therefore, like that beneath the eastern border of the 

 continent, must have been continued on from the period of general 

 fluidity. 



Moreover these Pacific-border eruptions took place in con- 

 nexion with, or as a consequence of, only a slight geanticlinal 

 uplift (that is, slight compared with the extent of the region, 

 although adding 10,000 feet to the height of the Rocky Moun- 

 tains, since the angle of slope made by it was not over 15'), 

 and through simple fractures that were unattended by nexings 

 or crushings of the region broken — conditions wholly incapable, 

 it would seem, of generating the heat required for so vast an 

 amount of subterranean fusion as the ejections indicate. 



Again, on both the Atlantic and Pacific borders of North 

 America, wherever the plications have been greatest, and the 

 conditions therefore favourable for producing the largest amount 

 of heat, there we find evidences of the profoundest metamor- 

 phism and of the least amount of fissure- eruptions ; and con- 

 versely, the regions of gentler plications and feebler metamor- 

 phism, or of none, are those of the most numerous fissures and 

 most abundant igneous ejections. The Green Mountains are 

 an example of the former, and the Triassico-Jurassic areas on 

 the Atlantic border, or the Tertiary and Quaternary outflows on 

 the Pacific slope, of the latter. The reverse should be true if 

 the heat for the fusion were transformed motion ; for fusion cer- 

 * Supra, page 211 et seq. 



