GEOLOGICAL SOCIETY OF LONDON. 
161 
about twenty miles in length from east to west^ and about ten 
from north to soutb^ and rising to the height of between 200 
and 300 feet above the sea. The tertiary strata of this island 
are_, for the most part_, deeply buried beneath a mass of drift^ in 
which lie huge erratic blocks of granite and other rocks which 
appear to have come from the north_, probably from the moun- 
tains of New Hampshire. The tertiary consist of Vv^hite and 
green sands^ a conglomerate,, white^ blue, yellow, and blood-red 
clays and black layers of lignite, all inchned at a high angle to 
the north-east, and in some of their curves quite vertical. They 
are finely exposed near Whilmark on the south-west side of the 
island, and in the promontory of Gay Head at its south-western 
extremity, where there is a vertical section of more than 200 
feet in height. 
Attention was first called to this formation by Prof. Hitchcock 
in 1823, who appears to be the only American geologist who 
has examined it personally. He compared the beds at Gay 
Head to the plastic and London clays of Alum Bay in the Isle 
of Wight, to which iithologically, they bear a striking resem- 
blance, consisting in both cases of variously and brightly colour- 
ed clays and sands with lignite, all incoherent and highly incli- 
ned. Various opinions, however, have been put forth as to the 
relative age of the Martha^s Vineyard strata which were assigned 
by Prof. Hitchcock, at a time when the tertiary formations of 
the United States were less known, to the Eocene period, while 
Dr. Morton supposed them to be in part only tertiary, and that 
they rested on greensand of the cretaceous period. The section 
at Gay Head is continuous for four fifths of a mile, the beds dip 
to the north-east generally at an angle of from thirty-five to fifty 
degrees, though in some places at seventy degrees. The clays 
predominate over the sands. In one place Mr. Lyell found a 
great fold in the beds, in which the same osseous conglomerate 
and associated beds of white sand, on the whole fifty feet thick, 
were so bent as to have twice a north-easterly and once a south- 
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