Geology and Natural History. 391 
nly i 
irregular masses, but sometimes shows curious and beautiful she. 
rescent crystallized forms, frequently observed elsewhere in similar 
rocks. It is interesting to observe that these peculiar dendritic 
forms are confined, as far as now observed, to the more hydrous 
of the trap rocks, although future study may not confirm this. 
nge 
older than the Tertiary. It is important to observe that the rock, 
as it Contains no hornblende, is not diorite, though that name has 
also been given to it. 
urning, however, from what may be called the normal rock, 
for example, that analyzed by Mr. Hawes just referred to, contain- 
ing almost no water, we find other varieties containing a consid- 
erable amount; and here the microscope comes to our assistance, 
making it possible to extend our observations over a wide range 
8 
the eastward, and of trap from other points south and west. 
we go from West Rock toward the eastern side of the Mesozoic 
t m a 
In addition to this massive, though generally chloritic trap, 
which makes up most of the great ridges laid down on Percival’s. 
