DEVELOPMENT OF TTIE EASTERN BORDER REGION. 39 



troughs, but overstepped their boundaries. This overstepping was true even 

 for the Appalachian region, and, consequently, rock-making areas of sub- 

 sidence were not then so narrowly limited in Eastern North America as they 

 were afterward. These early Paleozoic formations, the Cambrian and Lower 

 Silurian, spread from the interior continental seas across the lower parts of 

 the Archaean protaxis, filling the seas between the Archaean islands and 

 extending to the Atlantic border south of New York, and probably to the 

 Connecticut valley on the north. 



But after the Lower Silurian era had passed, and also the epoch of disturb- 

 ance closing the era, this overstepping the boundaries of the troughs in East- 

 ern North America was, in general, no longer a fact. The Upper Silurian, 

 Devonian, and Carboniferous rocks never extended over the Green Mount- 

 ains or beyond the Taconic range, for the region — that is, the Green Mount- 

 ain area — had, in the mean time, emerged. Moreover, it is not yet known 

 that these strata spread eastward from the Interior Continental area over 

 any part of the crystalline rocks of the Atlantic Border region, or, I might 

 say, over any part of the Atlantic Border region. They may and probably 

 do exist on the border beneath the Cretaceous and Tertiary, or beneath the 

 ocean's margin ; but they are not yet known from actual observation to have 

 extended east of the Archrean protaxis. The Jura-Trias of the Atlantic 

 border rests in many places on Archaean, Cambrian, or Lower Silurian, but 

 not as far as yet known on later Paleozoic rocks. According to Prof. G. H. 

 Cook, of New Jersey, borings through the Cretaceous formation between 

 New York and Trenton, N. J., reach only crystalline rocks, much re- 

 sembling those of New York island. 



The boundary-range separating the Interior Continental region from the 

 Atlantic Border region was, hence, greatly widened before the Upper Silurian 

 began, by the addition to the Arclmean of the Cambrian and Lower Silurian 

 formations, and their addition to a considerable extent in a crystalline or 

 metamorphic state. They were added in the metamorphic state in western 

 New England, where we have Lower Silurian and Cambrian strata in a 

 crystalline condition combined with the ranges of Archeean — those of the 

 protaxis — and all together in combination making up the Green Mountain 

 area as it existed in the period of the Upper Silurian. There is no question 

 as regards the Taconic system here involved. For the discoveries of fossils 

 by the Vermont survey, and by Wing, Walcott, and Dwight, have definitely 

 proved that the Archaean is bordered and combined in the Green Mount- 

 ain region with Cambrian and Lower Silurian strata ; and, being thus com- 

 bined, it was emerged before the Upper Silurian era began. The Archaean 

 protaxis of the Appalachian region was similarly combined with Lower 

 Silurian ; for uncrystallized Cambrian and Lower Silurian strata are visibly 

 so associated, and besides this, it is probable that part of the crystalline 



