96 GEOLOGY OP THE HIGH PLATEAUS. 



of diabase is given by some lithologists, but which are really dolerites and 

 basalts, bearing indications of a volcanic origin, and these are found as 

 contemporary or interbedded coulees. They differ notably, however, from 

 the intrusive diabases, though they are sometimes confounded with them. 

 In short, the ancient eruptives which remain as coulees have the volcanic 

 textures, and those which remain as intrusives have the granitic or some- 

 times the porphyritic texture, and the diorites and diabases equally with the 

 syenites and granites present no obstacle to Von Cotta's hypothesis, but 

 are to all appearances in full accord with it. 



It is as certain as anything in geological science can well be that the 

 texture of the granitoid eruptive rocks could not have been derived (at 

 least directly) from any special conditions existing prior to their eruption. 

 Every theory must presuppose that during their eruption or intrusion they 

 were plastic, and that a portion of their groundmass, if not the whole of it, 

 was amorphous and in a condition of igneous or aqueo-igneous fusion, and 

 in such a condition it is little less than absurd to suppose that any texture 

 at all resembling granite could have prevailed. The closely interlocked 

 crystals of such a groundmass are as antithetical to the very idea of plas- 

 ticity as it is possible to conceive. The crystalline texture must surely 

 have been a development altogether subsequent to plastic movement.* 

 There is, therefore, a lurking fallacy in the statement that granitoid rocks 

 had their periods of eruption in the earlier ages, while the volcanics' had 

 theirs in Tertiary time. The true and rational mode of stating the case 

 may be this : that through all the ages igneous magmas have been erupted, 

 which have, according to their final resting-places and the conditions there 

 existing, consolidated either into granitoid or half-crystalline rocks. The 

 magmas themselves have been the same in all ages, each to each within its 

 own group, and so too have the resulting rocks each to each under equiva- 

 lent conditions of consolidation. "We find in the Tertiaries only volcanic 

 rocks, because the corresponding granitoids are far beneath them and not yet 

 laid bare by secular erosion. We find among Pre-Tertiary eruptions chiefly 

 granitoids, because the corresponding volcanics have been swept away. 



* It is of course intelligible that some crystals may have existed in an amorphous fluent paste 

 during the eruption. 



