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 minei'als and the percentage of each in rocks. 



In those rocks which contain an unaltered base, 1 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 modem lithological methods. 



Cambridge, May, 1879. 



