12 DEPOSITS OF PHOSPHATE OF LIME. [bull. 46. 
These phosphate-bearing invertebrates appear to have been particularly 
abundant in the early Paleozoic seas. 
(4) Below the level of the Silurian the phosphatic deposits which have 
been worked probably belong altogether to the class of apatites or crys- 
tallized lime phosphates, and are probably all new deposits. They evi- 
dently occur through a large part of the Laurentian section, though, so 
far, the known deposits of economic importance are possibly limited to 
one portion of that vast series of rocks. 
The apparent absence of phosphatic deposits of economic importance 
in the section between the Devonian and the Cretaceous is remarkable. 
It is possible that it may be due to our lack of knowledge as to the 
chemical character of the deposits in those parts of the earth's crust. 
It is more likely, however, that such deposits do not there exist, owing 
to the fact that tbe invertebrate species of animals which secrete phos- 
phatic matter in their skeletons became relatively less abundant in the 
middle portion of the geological section • while the vertebrate species, 
the birds which accumulate guanos and the fishes which afford an abun- 
dance of bones and teeth to littoral deposits, as well as the mammalia 
whose skeletons occasionally form a considerable element in the later 
deposits, did not begin to contribute phosphatic matter to the rocks until 
comparatively modern times. 
The absence of phosphatic deposits in the Upper Paleozoic and Lower 
Mesozoic strata is well shown by the fact that, while in the Carbonifer- 
ous and the Triassic beds there are abundant laud surfaces which have 
been carefully explored, no phosphatic deposits of economic importance 
have been found in them, while on the relatively very limited areas of 
the Tertiary and Cretaceous formations where old land areas have been 
explored a large number of deposits of beds of nodular phosphate have 
been found. 
From the facts set forth in Dr. Penrose's report and the unpublished 
results of certain studies on swamps, we may draw certaiu general con- 
clusions as to the best method of prosecuting the search for unknown 
deposits of American phosphates. These conclusions are essentially as 
follows : 
First, as regards the superficial and recently formed deposits of phos- 
phates. We are driven to the conclusion that this class of deposits may 
reasonably be sought for wherever soft calcareous beds containing a cer- 
tain amount of lime phosphate have been subjected to long continued 
leaching by waters containing the share of carbonic acid gas which be- 
longs to all rain-water after it has passed through the mat of decayed 
vegetation. As loug ago as 1870 I became convinced that it was to the 
leaching out of the carbonate of lime by the carbonated water of the soil 
bed that we owe in the main the concentration of the nodular phos- 
phates of South Carolina. 1 Although it is still necessary to explain 
J See Proc. Boston Soc. Nat. Hist., vol. 13, 1871, p. 222. 
(48G) 
