Theoretical Considerations. 



243 



stone, at least to some extent. In some cases, however, there is rea- 

 son to believe that the abraded fragments were transported southerly. 

 Thus, the conglomerate in Greenfield and Bernardston, near the argillo- 

 micaceous slate, out of whose ruins it was obviously formed, (Nos. 137, 

 138, 139,) lies south of the parent rock : though a stratum of the slate 

 might once have extended farther south than at present, and have been 

 worn away by the powerful reflux wave which rushed easterly, when 

 the Hoosac range of mountains was elevated. The fragments of the 

 conglomerate of Mount Toby, and in Montague, correspond some- 

 what with rocks found several miles to the north, within the limits of 

 Vermont and New Hampshire : though it must be admitted that 

 these rocks also, might once have extended farther to the south than 

 they now do. 



It is an interesting enquiry, whether the greenstone ranges now ex- 

 isting in the very midst of the sandstone formation, were produced 

 anterior to that rock, or during the same epoch, or afterwards. In 

 all the lower beds of the sandstone formation, I have never found a 

 single fragment of the greenstone ; and, therefore, I infer that the 

 latter rock did not exist previous to the deposition of these beds. Nor 

 have I found any of the trap in the conglomerates of the higher beds, 

 except a narrow stratum, which I have described under the name of 

 trap conglomerate; and which lies in immediate contact with the 

 greenstone of Tom and Holyoke, so as to rest upon them. Green- 

 stone must, therefore, have existed in the vicinity before the formation 

 of this conglomerate ; and there is only one difficulty in supposing the 

 whole of the greenstone in the Connecticut valley to have been pro- 

 duced immediately subsequent to the deposition of the lower beds of 

 the sandstone. From some facts to which I have already alluded, 

 and which I shall describe metre particularly in speaking of green- 

 stone, it would seem that the upper beds of the sandstone, those that 

 lie even higher up in the series than the trap conglomerate, have been 

 disturbed and elevated by the intrusion of the greenstone. Hence I 

 should infer, that the greenstone continued to be produced, until nearly 

 all the beds of sandstone had become consolidated; and that probably 

 the existing trap ridges were the latest erupted. From all these 

 facts, then, I infer, that greenstone began to be erupted not far from 

 the middle of the epoch of the deposition of the sandstone; and con- 

 tinued to be thrown up at intervals, during the remainder of the pe- 

 riod in which the sandstone formation was advancing to its comple- 

 tion. 



