no Geology as a means of culture — A. Winchell. 
-vestigation; and this artistic acquirement is one of the forms of 
-culture for which the science of geology provides. 
The same demand for pictorial illustration leads the field ge- 
ologist to subsidize for his ends, the superb picturing power of 
the photographic camera. Topography, mountain forms, rock- 
structure, details of stratification, water-falls invite to the appli- 
cation of the camera while in the field; and the exact delinea- 
tion of fossil forms is greatly promoted by photography in the 
laboratory. Thus the geologist is led still further to diversify 
his accomplishments, and add to the sources of his efiiciency as 
a geologist, and of his enjoyments as a lover of nature. 
These various forms of mental exercise and discipline are in- 
cident to the acquisition of the facts and doctrines of geologic 
■science. I have illustrated a higher range of geological truth, 
and I wish to impress the fact that its acquisition calls into ex- 
ercise another range of intellectual powers. The faculties of 
deductive or a priori reasoning come into play in the attempt 
to proceed from an admitted principle to the particulars which 
it involves or necessitates as consequences. Geological investi- 
gation very freq?ie?itly takes the deductive form. It does not 
often proceed from necessary principles, as in mathematical rea- 
soning; but generally from a principle or truth established by 
previous inductive research. When a distinguished American 
geologist described a large number of three-toed tracks found in 
the brown sandstones of the Connecticut valley, and ascribed 
them to extinct species of birds, the elder Agassiz reasoned de- 
ductively when he declared that they could not be bird-tracks, 
since birds, according to all inductions, had not begun to exist 
at so early an age of the world. Similarly, the geologist de- 
clares that coal will never be discovered in the valley of the 
Hudson river, however black and misleading some of the slates 
may be; since all productive coal measures have been found to 
hold a higher stratigraphic position. More marked and pro- 
longed employment of deductive inference is observed in the 
treatment of those geological problems which admit of the ap- 
plication of the methods of mathematical analysis. Some of 
these problems are as follows: The temperature of the earth's 
interior; the thickness of the earth's .crust ; the condition of the 
central matter of the earth; the existence of tidal effects in the 
