30 Fhe American Naturalist. [January, 
sil, and many fragile and delicate organisms, such as the fossil 
hydrozoans, known as graptolites, are retained in the fine- 
grained slates (which have originally been mud layers) that 
would have scarcely survived comparison in the coarser and 
impressionable beds of sand. Such muddy layers may be en- - 
tirely argillaceous or markedly siliceous, or they may be cal- 
careous and formed at considerable depths, as in the case of 
the deep-sea ooze which assumes the character, as described 
by Sir C. Wyville Thompson, of a grayish, calcareous paste. 
These beds from this fine state of mechanical division are pre- 
cisely adapted for keeping unbroken the tests, coverings and 
hard parts of the animals that are buried in them, and if 
sufficiently argillaceous to prevent crystallization, upon con- 
solidating into stony strata retain their contents in a very 
beautiful and perfect condition. 
The deeper zones of the sea nurture the coral growths, or 
receive from the pelagic life above them the unceasing con- 
tributions of dead shells, the cases of foraminifera, and the 
skeletons of aberrant crustacea, or form beds congenial to 
glassy sponges, submarine thickets of crinoids and fields of 
gorgonias. These sea-deposits, which are somewhat exempt 
from the mingling sediments of the shore, though, of course, 
only approximately, and more or less completely, according to 
the nature and distance of the neighboring coasts, as regards 
their fossiliferous character, form very perfect beds of deposi- 
tion. The variety of animal life becomes here very great, and 
its fertility continually augments the rising sheets of animal 
precipitation. The pelagic life above these regions is con- 
stantly contributing its mineral contents to these beds, and 
the broken, half-dissolved shells of pteropods, with the tests 
and insoluble residue of foraminifera, form a calcareous com- 
mixture, in which whole shells, corals, crinoids, star-fishes, sea- 
urchins and the dust raining down from dead and decomposed 
swimming organisms, parts of fish, etc., become imbedded. 
The explorations of the Challenger showed that, according to _ 
Murray, the foraminifera of the open sea are subjected to solu- 
tion in the carbonated sea-water, and that their argillaceous . 
ash, so to speak, drops down and spreads upon the floor of the 
