154 The American Geologist. September, 1894 
comminution of foraminiferal shells; while standing out con- 
spicuously amid the remains of these minuter organisms are 
comparatively large calcareous spicule-like bodies, which, on 
investigation, turn out to be the disassociated rods that made 
up the external prismatic layer of shells of Inoceranms. This 
outer layer in Znoceramus problematicus seems invariably to 
have become detached from the inner nacreous layer, and, al- 
most without exception, it has become completely disintegra- 
ted into its constituent prisms. And so it happens that, in 
the Inoeeramus-bearing beds of the deposit, the soft matrix 
in which the only remaining portions of the shells, the na- 
creous portions, lie imbedded, is composed very largely of 
these rod-like prisms mingled with skeletal parts of the ordi- 
nary chalk-forming organisms. Even in the typical, massive, 
chalky beds that bear no outward traces of mollusk shells of 
any kind, these same prisms, while greatly diminished in 
numbers, are by no means uncommon; and so in any consid- 
eration of the constituent elements of the chalk these sepa- 
rated units from the external layer of valves of Inoceranms 
must be reckoned as no unimportant factor. 
When the chalk is treated with acid there remains a small 
amount of insoluble matter consisting of clay, fine grains of 
sand, a very few minute pebbles, none of which, so far as ob- 
served, exceed five millimeters in diameter, and a small num- 
ber of internal casts of the chambers of Foraminifera. Nearly 
all the foraminiferal shells have the chambers filled with cal- 
cite ; a few have these cavities still empty or filled simpty 
with air; but in a small number of cases the chambers were 
filled with an opaque, insoluble mineral, probably silica deeply 
stained with iron oxide, that remains as perfect internal casts 
after the shell has been dissolved in acid. The amount and 
composition of the residuum varies with the purity of the 
chalk. In the purer samples it scarcely exceeds one per cent.. 
in others, of course, the percentage ranges very much higher. 
The Micro-paleontology ok the Chalk. 
In all the chalk examined the Foraminifera are very numer- 
ous. Many are large, vigorous looking specimens of the types 
to which they belong, and an unusual number of the shells 
remain perfect. They are easily separated from the finer par- 
ticles constituting the matrix by carefully grinding the chalk 
