MUSEUM OF COMPARATIVE ZOOLOGY. 287 
cal elements between two points not in juxtaposition is very slow in 
such a pasty mass. The crystallization will then be imperfect, as will 
also be the removal of the chemical elements from the base in rapidly 
cooling magmas. These facts, together with the diverse origin of the 
rock minerals, have served to render futile all attempts to determine 
by chemical analysis the minerals and the percentage of each in rocks, 
In those rocks which contain an unaltered base, I find that the base 
is a surer index of the chemical composition, if we must be restricted 
to one thing, than the enclosed minerals are. 
Exact field, microscopic, and chemical investigations are needed upon 
the vein-stones, ore deposits, and sedimentary rocks, as well as upon 
the eruptive ones, — work which promises a fruitful harvest for the 
earnest laborer. If the signs of the times are read aright, the day 
is not far distant when the battlefield of that all-embracing science, 
geology, will be transferred to the domain of petrography ; and no young 
geologist who desires to stand in the front can afford to be ignorant 
of the modern lithological methods. 
CAMBRIDGE, May, 1879. 
