1875.] 489 [Staler. 



floor of this synclinal gradually rises as we go to the southwest, until 

 near the Rhode Island line the depression ceases. 



In Narragansett Bay we probably have a precisely similar furrow 

 coming in echelon order a little to the east of that in which Boston 

 bay lies; its northernmost point lies a little to the north of the most 

 southerly part of the Boston trough. This arrangement will be rec- 

 ognized by those familiar with the Appalachian Chain as quite in 

 the order of its structure. The great downthrow on these faulted 

 folds results in imprisoning within the lower crystaline rocks a 

 great thickness of Palaeozoic strata. To this protection we owe the 

 preservation of a great thickness of the rocks between Cambrian 

 and the Upper Carboniferous, which have been lost over the surface 

 where they were exposed to the intense erosion of the successive 

 glacial periods that swept this shore. The Artesian well bored by 

 the Boston Gas Company, penetrated for seventeen hundred and 

 fifty feet through the Cambridge slates without finding the bottom. 

 It is evident, therefore, that if I am right in regarding these de- 

 pressions as great faulted down-folds, they had a magnitude of 

 dislocation quite commeasurable with the greatest of Appalachian 

 folds. 



That there should be an eastern outlier of the Appalachian sys- 

 tem of the age of its Pennsylvania section might seem, on general 

 principles, pretty doubtful. There is no doubt, however, that the 

 dislocation lines of our coast correspond generally with the direction 

 of that chain. There is also good evidence of the occurrence of sub- 

 marine ridges essentially parallel with this system. So it does not 

 seem to me unreasonable to interpret the facts as I have done. 



The question will arise, and I propose to discuss it at some extent 

 hereafter, how these ridges of the height of thousands of feet have 

 lost their relief since the Carboniferous period, while the similar 

 ridges of the Alleghanies have not been anything like as much 

 eroded. This is easily answered by supposing that this region has 

 been repeatedly subjected to glacial wearing, giving an erosion rate 

 many times greater than in the Alleghanies, where the glacial ero- 

 sion has not been so severe. The whole form of the Boston fjord 

 shows profound glacial wear working in the relatively soft rocks of the 

 Palaeozoic series, and the evidence of glacial erosion in Narragansett 

 Bay is almost as great. Taking an erosion rate of one foot in ten 

 thousand years (probably within the limit in this region), it requires 

 but thirty million years to take away three thousand feet from this 



