342 STRATIGRAPHICAL GEOLOGY. 



to each other, and situated in the same ancient valley of depression. 

 The animals, when living, occupied the same arm of the sea, and were 

 separated from New York by the long Green Mountain ridge. 



Extent of the Helderberg Sea. The many localities of Helderberg 

 rocks scattered over the middle Atlantic region indicate the presence 

 of the ocean in very many parts of it. At Littleton some of these rocks 

 now rise at least 1800 feet above tide- water. If the region rose and sank 

 in close correspondence with its present surface, it would be compara- 

 tively easy to map out the limits of the land and water. The impression 

 gains upon me that this depression occupied certain great valleys, leav- 

 ing, as hills between, the areas that are now in many places lower than 

 the uplifted Helderberg. The Connecticut would have been one of these 

 areas of submergence, the Memphremagog a second, nearly parallel, sup- 

 posing that the first named followed the course of the Upper Ammo- 

 noosuc and the Androscoggin lakes into Maine. Littleton village must 

 have been at least 2000 feet lower than now. The low region of the 

 Merrimack basin has not yet afforded any evidence of a Helderberg sub- 

 mergence. More likely it constituted an elevated island at the time of 

 the growth of the Grafton county coral reefs. 



Until recently I had supposed the Blueberry mountain range consisted 

 of rocks even newer than the Silurian, anticipating future paleontological 

 discoveries of Devonian life. For reasons given above, it is probable 

 that these slates, sandstones, and conglomerates should be placed with 

 the Cambrian, a much older formation. That mountain range may have 

 been a submarine ridge, with the coralline growth upon both sides, as is 

 proved by the occurrence of the fossiliferous structures along the two 

 diverging lines. There is a very marked want of conformity between 

 the coralline and Huronian layers, while there is not so much difference 

 between the Helderberg and Cambrian. The former, closely pressing 

 the latter, would naturally exhibit the same line of strike; and the near 

 verticality and inversion of the strata, the force of elevation having been 

 greater in some areas than others, render it difficult to distinguish an 

 unconformability through the dip. 



