428 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
high water, and are exposed during seasons of low water. The 
constant repetition of this process has dissolved out the matrix in 
which the fossils were embedded, and which was only very slightly 
more soluble, and has left the fossils in relief. Although many of the 
fossils are well preserved, the majority are very poorly preserved and 
cannot be determined with any assurance. 
From the nature of the Sutton limestone it is seen that it is of 
marine origin, and formed largely from the remains of various calca- 
reous organisms, chiefly corals and pelecypods. Such, doubtless, is 
the origin of the other numerous limestone lenses of the Vancouver 
group. It is to be presumed, therefore, that the volcanic rocks were 
largely submarine, and the rarity of tuffs and agglomerates, and of 
interbeds of terrestrial material adds weight to this conclusion. It 
is probable that some of the old vents were above sea level since a 
few conglomerates composed of rounded volcanic fragments and 
interstratified with tuffs are known. The old vents were probably 
islands and it was on these volcanic islands, far removed from the 
mainland to the east, that the organisms which built the coral reefs 
and coquina beds lived. 
