1849.] DESOR AND CABOT ON NANTUCKET. 343 



Davis's view, the shells are deposited in this way by the tidal current 

 along the shoal ridges, which act as so many nuclei. 



Until last year it was assumed by the geologists of this country 

 that there were no fossils to be found in the drift, south of Lake 

 Champlain and the State of Maine, when one of us had the good 

 fortune to discover several species in the drift of Brooklyn near New 

 York*. Similar fragments, especially of Venus mercenariay have 

 since been found in the cliffs of Point Shirley, in Boston Harbour. 

 Now, as the fossil shells in both places are of the same species as 

 those of Sancati Cliff, there is every reason to consider them as be- 

 longing to the same period, their more or less perfect state of preser- 

 vation depending merely upon local influences. It ought further to 

 be stated, that wherever the shells are worn or broken, and the strata 

 which contain them coarse and irregular, it is either in such places 

 where the tidal currents must have been violent, so as to carry and 

 deposit promiscuously heavy pebbles and minute shells, as in the 

 cliffs of Point Shirley ; or in such places where we must suppose that 

 floating ice was at work, carrying indiscriminately heavy materials, 

 pebbles and boulders, together with oysters and other shells detached 

 from the neighbouring flats, and heaping them up in the corners of 

 bays and sounds. This seems to have been the case with the coarse 

 deposits of Brooklyn, where oysters and Venus are generally found 

 imbedded in a reddish loam intermixed with pebbles and boulders, 

 many of which are distinctly scratched, thus reminding us of similar 

 actions which you have described in Fundy Bay and in the St. Law- 

 rence ; whilst in other places, like Nantucket and the bays and fiords 

 of Maine, a more quiet action prevailed, so as to allow the shells to 

 be preserved in their natural place and position after death. [Among 

 the drift fossils collected at Augusta, on the Kennebee, there are 

 many which have preserved tlieir colour in a most perfect manner, 

 especially Astarte and Mytilus.] 



Finally, the fossils of the drift of Nantucket bear such a striking 

 similarity to those of the newer pliocene of the Southern States, 

 that they become a natural link between the northern and southern 

 deposits. Instead of considering these as so many distinct formations, 

 we should therefore henceforth look at them as mere modifications of 

 the same deposit, being the result of the same agencies, viz. oceanic 

 tide-currents along the whole coast of the United States, combined 

 with gradual and secular oscillations of the whole continent, the local 

 strength of the tidal currents affording a suflicient explanation for 

 local diversity in the arrangement and size of the materials in each 

 locality. 



Viewed in this light, the drift formation, heterogeneous as it may 

 appear at first, can be cited as a fair illustration of that great principle 

 of yours, that it is by the application of actual causes that we arrive 

 at a true understanding of the origin and diversities of geological 

 formations. 



* E. Desor's Letter to M. de Verneuil, in the Bulletin de la Societe Geologique 

 de France, 1847. A most interesting collection of the drift fossils of Brooklyn 

 has since been made by Mr. Redfield. 



