504 The Geological Museum of the School of Mines, [ August, 
we find the bones themselves but little altered from their original 
condition. At other times the organic matter of the speci- 
men, a piece of wood, a bone, a shell it may be, have been 
replaced by silica so as not only to retain the general form, but 
even the most minute structure of the original substance. Such 
a replacement is called petrifaction. Wood is frequently thus 
petrified so as to preserve its microscopical structure as perfect as 
it was when the plant was yet in leaf. Again we may find but 
the impression of a fern or of a fish, made in soft mud or sand, 
which has been hardened into rock and has faithfully preserved 
the form of the frail body that perished ages ago. The plastic 
mud along the shores of bays and rivers is frequently trodden by 
animals or pitted by falling rain-drops ; such a surface by becom- 
ing covered by a layer of sand or mud may retain for indefinite 
ages the inscriptions thus impressed upon it. In these and many 
other ways, the life-history of distant ages has been written on 
' the rocks and preserved to our own day, with an accuracy and 
fidelity which cannot be too highly appreciated. 
The great interest connected with the first appearance of life 
on the globe is indicated by the prolonged discussion that took 
place in reference to the organic nature of the eozodn, which, as 
far as at present known, is truly the “dawn animal” of the world. 
Specimens of this interesting fossil are contained in the first case 
at the northern end of the geological hall. Now that we have 
made the first step in our journey through the geological ages as 
here arranged, we will pass slowly down the long row of cases, 
and in doing so, review hastily the life-history of the earth. 
The Eozoön belongs to the lowest sub-kingdom of animal life, 
the Protozoa, which also embraces our familiar sponges, the struc- 
tureless amoeba, etc. The case containing the Eozodn shows us 
also the forms of life that followed this humble beginning. These 
are the fossils of the Silurian age, or the age of mollusks, as it is 
sometimes called in reference to the great abundance of the 
remains of “ shell-fish,” which far outnumber all the other fossils 
of this formation. The collection contains six thousand speci- 
mens of this ancient fauna, which were all embraced in the first 
four sub-kingdoms of animal life. The Protozoa are represented 
by the Eozoon, sponges, receptaculites, etc.:—the Radiates by 
| corals, crinoids, and star-fishes. The Mollusks, as we have men- 
=» tioned, were in great force, as the fossil shells testify. The 
