298 0. C. Marsh — Geology of Block Island. 



The presence of such great masses of stratified clay, evi- 

 dently of high antiquity, on this diminutive island facing the 

 Atlantic, opens up many questions of interest beyond the mere 

 geological age of the deposits. These beds of clay must be 

 the remnants of a great formation, vi^hich extended out far 

 beyond the present coast line, and being of fresh-water origin 

 and laid down in quiet waters, they prove the former existence 

 of an extensive barrier along the continental border between 

 them and the Atlantic depths beyond. 



It has long been a reproach to American geologists that they 

 could find no true Jurassic on the Atlantic coast, although 

 the next formations above and below are well developed at 

 many points. May not the missing strata be represented in 

 the characteristic series of Block Island clays ? The evidence 

 as it stands to-day points to this conclusion. 



New Haven, Conn., September 5, 1896. 



Postscript. 



Since the above was in print, I have visited Long Island 

 and Martha's Yineyard to ascertain if the Block Island clays 

 are represented there, as they should be if my conclusions 

 are correct. On Martha's Yineyard, I found that the great 

 series of variegated clays forming Gay Head, and generally 

 regarded as Tertiary, are certainly Mesozoic, and all apparently 

 Jurassic. They have the same general features as the Block 

 Island clays, but are more highly colored, and have been more 

 disturbed. They, also, suffered much from denudation before 

 the Glacial period. 



Prior to that period, however, they were covered by a series 

 of marine Miocene deposits, remnants of which still rest uncon- 

 formably upon them. The vertebrate fossils reported from 

 Gay Head were apparently all from this horizon, which also 

 contains many mollusks. 



The cliffs of Gay Head are rapidly washing into the sea, 

 the base being first undermined by storms, causing very 

 extensive landslides which have led to much confusion in 

 regard to the natural position and mutual relations of the 

 various strata. 



On Long Island, I visited a number of localities, including 

 Montauk Point, where there are clay beds similar to those 

 on Block Island, although possibly of somewhat later date. 

 Other preglacial freshwater clays at various points on the north 

 shore appear to be of the same age as those at Gay Head. 

 I hope soon to discuss this subject more fully elsewhere. . 



New Haven, Conn., September 21, 1896. 



