234 ^^^^ American Geologist. April, 1904. 
organisms themselves, such as is going on in the formation of 
the true organic rocks. 
Turning now to the biogenic or organic rocks, we find no 
pure organic salts of the alkali metals,* and hence our first col- 
umn (a) remains a blank under this heading. Calcareous de- 
posits, (b), however, are well represented, and it is here that 
the main source of calcareous deposits of both endo- and exo- 
genetic types is to be sought. We may readily distinguish two 
groups, the unassorted or massive, and the assorted or strati- 
fied. The former include the reef rock formed by the growth 
in situ, of corals, Bryozoa, hydrocoralines, calcareous Algse, 
etc. The latter comprise : 
1. Shell beds, such as deposits of unbroken mollusk or bra- 
chiopod shells, echinoderm tests ; the larger Foraminifera, etc. 
2. Shell oozes, like chalk, Globigerina ooze, pteropod ooze, 
etc., and the limestones resulting from their solidification. 
3. Crinoidal limestones, composed chiefly of the stems of 
crinoids which become dissociated on the death of the animal, 
and 
4. Certain oolytes and other stratified deposits due to lime- 
secreting Algae, and which are free to assume stratified charac- 
ter. 
While all of these deposits may and frequently do occur in 
great purity, with the frail shells composing them scarcely brok- 
en, it is nevertheless true that they are very commonly in part 
composed of broken material, and thus approach the clastic 
condition. In fact it is here that we generally find an imper- 
ceptible transition to the clastic deposits (calcarenytes and cal- 
cilutytes) without being able to draw a line of distinction. Nor 
is it always possible after diagenesis, or alteration by static me- 
tamorphism, has set in, to determine the origin of a given rock 
mass. 
Besides the carbonate of lime, the phosphate must be placed 
here. Such deposits may be derived from the accumulation of 
phosphatic shells, like those of certain brachiopods (Lingula) 
and of exoskeletons like those of the trilobites and other Crus- 
tacea, of which not inconsiderable accumulations are at times 
found. More pronounced than these, however, are the deposits 
• The large percentage of ammonia salts in guano (31 per cent) which may- 
even be greater than the phosphate of lime (22.5 per cent), suggests that this 
organic deposit is in part at least of this group. 
