454 Geology as a Science and an Art, [August, 
very important, and partly because I want to make it clear 
that we cannot expedt mathematical exactness in such a 
science as Geology, which it must be remembered belongs 
to our first class. In Geology, in fadt, and in the allied 
sciences, the advance of our knowledge has, perhaps, been 
greater in the direction of organisation than in that of 
exactness. And with respedt to this increased organisation 
it may be noticed that not only have geologists been able to 
class their stratified deposits, according to their mode of 
origin, into rocks formed by (x) mechanical agency, (2) by 
chemical agency, and (3) by vital agency ; not only have 
they ascertained the order of the sequence of the strata ; 
not only have they begun to grasp the definite succession of 
life-forms ; but they have traced the shading of the stratified 
deposits into metamorphic rocks, these again into plutonic 
rocks of the granitic type, and these once more into rocks of 
volcanic origin. 
But though geological knowledge is, compared to Mathe- 
matics, inexadt, it does nevertheless become more exadt, as 
any one may see who will take the trouble to compare our 
present knowledge with that of half a century ago ; but we 
must not expedt mathematical exadtness. As we shall see 
diredtly, Geology is an historical science ; it deals with the 
past. And if we cannot apply mathematical processes with 
success to the more complicated of the phenomena of to- 
day, the conditions of which are actually present, still less 
can we apply them to the past, when the conditions have to 
be inferred. 
There is a well-known controversy between geologists and 
physicists, in which the distindtion between the two kinds of 
science should be steadily borne in mind. The discussion 
is on the question of geological time. The physicist says 
to the geologist “ The earth’s crust solidified and cooled so 
many years ago,” and brings forward in proof certain defi- 
nite figures obtained by mathematical processes. “ Geolo- 
gists,” he says, “ must confine their speculations to this 
period.” The geologist answers, “ I can’t bring forward 
figures, but that period is not nearly long enough.” There 
the matter rests to-day, and, dodtors differing, the general 
public does not know what to think. But I fancy that the 
potency of figures inclines them to the view of the physicist. 
Now the point to which I wish to draw attention is this — 
that it is the business of the student of physics, dealing as 
he does with a science of our second class, to isolate certain 
fadts by excluding or allowing for all interfering causes. He 
considers each of the fadtors of phenomena separately ; he 
