608 THE JOURNAL OF GEOLOGY. 
moulding the surface. 4) The superadded agency of life. Ot 
these causes, the first is the molecular power by which the material 
of the crust has been prepared. The third and fourth have only 
worked over the exposed surface. But the second while molec- 
ular in origin, is mechanical in action, and in the way of con- 
traction, especially, it has engaged the universal sphere, causing 
a shrinkage of its vast sides, a heaving and sinking in world-wide 
movements. Its action, therefore, has been coextensive with the 
earth’s surface through the earth’s history” (loc. cit, p. 340). On 
a later page a footnote again refers to this same dominant idea: 
‘‘T have alluded on a former page to an analogy between the 
progress of the earth andthat of a germ. In this there is nothing 
fanciful; for there is a general law, as is now known, at the basis 
of all development which is strikingly exhibited even in the 
earth’s physical progress. The law, as it has been recognized, is 
simply this:—Unity evolving multiplicity of parts through suc- 
cessive individualizations proceeding from the more fundamental 
onward” (p. 346). 
Notwithstanding all the additions of details and statistics in 
illustration and elaboration of this idea, we see, up to the last, 
this is the dominating principle about which his system of geology 
was built; and the American continent, as its geological features 
were gradually opened to light, was recognized as the most typ- 
ical illustration of this system to be found upon the globe. In 
the last edition of the Manual we find these words: ‘‘North 
American geology is still its chief subject. . . . The idea long 
before recognized [7. ¢., before 1855 | that all observations on the 
rocks, however local, bore directly on the stages in the growth 
of the continent, derives universal importance from the recogni- 
tion of North America as the world’s type-continent—the only 
continent that gives, in a full and simple way, the fundamental 
principles of continental development.” 
He was not, however, carried away by theories, his scientific 
research was always deep, thorough and exact. As he was pre- 
paring the report on the geology of the exploring expedition 
he was not satisfied with simply describing what he saw. He 
