shalkr.] INTRODUCTION. 11 
logical conditions of the Laurentian area exist iu the Adirondack dis- 
trict and in the southern parts of the Appalachian system as well as in 
several districts of the Rocky Mountains. It would be remarkable if ex- 
tensive deposits of this nature, so common in Canada and iu the equiv- 
alent rocks of northern and southern Europe, should not be found at 
many points in our American Archaean formations. It is on this account 
that so much space iu this report is given to the description and illus- 
tration of the Canadian apatite deposits. So, too, we may hope to find 
in the ancient rocks of this country deposits analogous to the great 
Logrosan and Caceres veins in the province of Estremadura, Spain. 
The Cretaceous deposits of Belgium (which at the present time are, 
next after the phosphate beds of South Carolina, the most productive 
in the world) present a type of beds not yet found in paying quanti- 
ties in the United States, though deposits of the same age, formed 
under about the same conditions, abound in this country. It is uot 
to be expected that phosphatic deposits will exactly repeat themselves 
in strata of the same age in widely separated regions ; yet it is clear 
from the summary account of the geological distribution of these phos- 
phates in Europe and North America that in the case of these, as well 
as in that of other substances of value in the arts, there are certain 
guiding principles which we may base on the stratigraphy of the de- 
posits to aid our search. The known workable deposits of a phosphatic 
nature are limited to certain portions of the geological section. Begin- 
ning at the surface of the deposits now forming, these zones are, iu 
descending order, as follows : 
(1) Superficial deposits, iucludiug (a) those formed in the manner of 
guanos; (b) the deposits formed in the bottoms of fresh-water swamps, 
sometimes in connection with deposits' of bog iron ore (hematites); and 
(c) deposits which are the result of the long continued decay of rocks 
containing a small portion of lime phosphate intermingled with lime 
carbouate, as, for instance, the deposits of North Carolina. This super- 
ficial group of deposits has no other common feature save that they are 
on the surface and are due to causes now or recently in action. 
(2) Deposits of the Tertiary and Upper Cretaceous. These deposits 
are generally the result of reactions which took place on ancient land 
surfaces, the phosphatic matter being such as formed in swamp beds 
or in ablation deposits like those of the Carolinas or ot eastern England. 
Below the level of the Cretaceous no important deposits of phosphate 
have been found in the vast section of rocks which lies between that 
era and the Devonian horizons. 
(3) In the horizons below the level of the Upper Silurian bedded rock 
phosphates and apatite deposits occur. These infra-Devonian bedded 
rock phosphates seem to have derived their phosphatic matter from the 
animals, brachiopods and small crustaceans, which separated that sub- 
stance from the sea insects or other food which the old oceans afforded. 
(485) 
