140 GEOLOGICAL RECONNOISSANCE 
attention to this, the geologist would, dispense with one of his principal 
aids in drawing important practical inferences and proofs corroborative of 
his assertions. This branch of the survey might appear to some, without 
due reflection, a matter of little importance; but when we consider that 
it is the surest and safest guide to the identification of. all the formations 
of sedimentary origin, and even of the individual members of such for- 
mations, as for instance, the beds of coal, we discover that it is the index, 
the criterion, the sign, the “ divining rod,” if I may so express it, which is 
to lead the geologist in his search after mineral wealth. 
I cannot present the practical importance, as well as the interest con- 
nected with the study of these relics of bygone ages, in a stronger light 
than by inserting here an extract from my report of the surveys made in 
behalf of the United States, some years since, in the North-west: 
“‘ The study of the organic remains of rocks is, indeed, a most beautiful, a 
most fascinating research. What can be more extraordinary: that. we, 
the generation of the nineteenth century, should exhume froni out the 
hard substance of the solid rocks, the delicate forms of organic beings of 
bygone ages, and display to the wondering eye of the naturalist, even 
their minute anatomical details? And this, not alone of races which 
inhabited this earth in times immediately preceding the human epoch; we 
are even permitted to contemplate, and restore to our perceptions, the 
very fishes, mollusks, and corals, that swarmed in the carboniferous seas 
millions of ages ago. The animal matter composing their tissues and 
bones is indeed gone, but the simultaneous mineral infiltrations preserve 
a perfect counterpart. We can depict those remarkable and elegant forms 
of vegetation which constituted the forests, that fringed the shores of that 
same treacherous and overwhelming ocean. We seize them in the very 
act of uncoiling their frond, and unfold to the admiring gaze of the 
botanist, that luxuriant canopy of foliage that once waved in the sea- 
breeze nurturing their stems. We accomplish even more than this: we 
can read the records of myriads of the lower orders of animals, that date 
their existence yet further back than the times that gave growth to trees, 
now stored up as mineral fuel in the bowels of the earth—to times at least 
as long prior to the coal formation, as that geological era is antecedent to 
the present time ; we can assign to each its place in the zoological systems, 
and fill up the gaps in the existing orders of the animal and vegetable 
kingdoms. 
“ To think that we, at this day, can demonstrate the structure of the eye 
of some of these—the most ancient races—and even count the lenses by 
which light was concentrated to the optic nerve, is truly astonishing! Is 
