414 



Scientific Geology. 



not great enough to complete the fusion. Or rather, may it not he 

 probable, that the perfect fusion of the rock out of which these un- 

 stratifled ones were produced, gave rise to the granite; while those 

 portions that were not so entirely fused as to admit of entirely new 

 and perfect combinations and crystalizations, might have formed those 

 portions of the rock which I call greenstone, although some of it 

 might as well perhaps be denominated sienite. I am aware that it is 

 not yet well ascertained, how the same materials should at one time 

 have produced granite, at another sienite, at another porphyry, and at 

 another greenstone. But some other facts which I have noticed on 

 this subject, and which will be detailed in speaking of granite, render 

 it somewhat probable that the more or less perfect fusion of the mate- 

 rials may have been the principal cause. According to this hypothe- 

 sis, we might explain how it happens that greenstone and sienite for 

 the most part, were produced since the formation of granite. For ge- 

 ology furnishes abundant evidence that the temperature of the interior 

 of the earth has been gradually sinking, since the commencement of 

 these processes. And then again, the later any rock was erupted the 

 less chance it has had for undergoing a second fusion, which, it may 

 be, is all that is necessary to convert it into some older variety of rock. 

 However, I will not dwell upon a suggestion that is so very hypo- 

 thetical. 



As we proceed farther from Boston, the sienite increases and the 

 greenstone decreases in quantity, and we begin to find granite desti- 

 tute of hornblende, which at length often becomes extremely coarse; 

 as in Billerica, Andover, &c. The greenstone, however, occasional- 

 ly appears associated with the perfect granite, as with the sienite ; 

 though I do not recollect any instance where the passage from the 

 greenstone to the granite is gradual, as is the case between green- 

 stone and sienite. Generally the greenstone forms veins in the gran- 

 ite. I have sometimes traced them not more than a foot or two wide 

 for several rods, (as in Weymouth,) retaining their direction and 

 width with almost mathematical exactness. 



In the manner that has now been described, is the greenstone of 

 the eastern part of the State intermingled with its unstratified associ- 

 ates, as the youngest member of the group. To mark out the pre- 

 cise limits of this rock in that section, would require immense labor, 

 both on account of the great quantity of diluvium that overlies the 

 rocks, and the difficulty of drawing the line in all cases between green- 

 stone and sienite. Nor, if it be correct that all these unstratified rocks 



