0. C. Marsh — Geology of Block Island. 295 



Art. XLY. — The Geology of Block Island; by O. C. Marsh. 



A RECENT visit to Block Island gave me an opportunity to 

 examine its geological structure, and this proved much more 

 interesting than the published accounts had led me to expect. 

 1 had previously seen this island only from a distance, yet I 

 supposed from what I had read that it contained none of the 

 older rocks in place, but was a remnant of the great terminal 

 moraine that, in the glacial period, was pushed over from the 

 main land of southern J^ew England, and left its debris as Long 

 Island and the other islands to the eastward.* That Block 

 Island was once connected with Long Island is suggested by a 

 glance at a map of the New England coast, and that the same 

 great moraine extended over both is equally evident from facts 

 well known. An examination of Block Island itself, however, 

 soon proved to me that these glacial deposits were merely a 

 superjficial covering, while the main body of the island was 

 formed of much older beds, the exact age of vrhich offers a 

 most interesting problem. 



These lower strata consist mainly of massive beds of clay, 

 more or less arenaceous, and all considerably disturbed. The 

 general inclination is to the northeast, as is well shown in the 

 line sections exposed on the coast, especially in the bluffs on 

 the east and south of the island, which are rapidly wearing 

 away from the assaults of Atlantic waves. Some of these 

 clays are white in color, as seen in the cliffs at Clay Head, 

 on the northeast shore of the island. Some bright-red clays 

 also occur near the water's edge at low tide, at the foot of the 

 same bluff, but the most of these deposits are gray or brown 

 in color. At a few points, where carbonaceous matters have 

 discolored them, they are nearly or quite black, and in such 

 places, specimens of fossil wood and other plant remains are 

 comparatively abundant. One locality of this character may 

 be seen in a bluff on the southeast shore, a short distance west 

 of the Ocean Yiew hotel. Another marked character of these 

 clay cliffs is the deposits of iron they contain. This ore is 

 mainly limonite, usually in thin seams, and its decomposition has 

 stained the layers in many places a rusty color, which frequently 

 serves to indicate the dip of the strata. Iron pyrites occurs in 

 the dark clays with the plant remains. 



These massive beds of clay, with their characteristic features, 

 are well shown in all the high cliffs of the coast that are being 

 rapidly eroded by the ocean storms. The famous Mohegan 



* Seventh Annual Report U. S. Geol. Survey, p. 304, 1888; and Bulletin U. S. 

 Geol. Survey, No. 53, p. 11, 1889. 



Am. Joue. Sci. — Fourth Series, Yol. II, No. 10. — October, 1896. 

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