ORIGIN OR THE HARD ROCIC PHOSPHATES. G1 
The boulders of silica are, therefore, masses of silicified lime¬ 
stone, the fossils originally present in the limestone having for 
the most part retained their form. 
PHOSPHATE BOULDERS. 
The phosphate boulders are formed either by replacement of 
the limestone or by precipitation from solution. 
PHOSPHATE BOULDERS FORMED BY THE REPLACEMENT PROCESS. 
Some of the phosphate boulders and pieces of rock are evi¬ 
dently formed by the replacement of the carbonate of the original 
limestone by phosphate. That this is true is proven by the fact 
that the shells and other fossils that made up the original lime¬ 
stone are sometimes well preserved, and from these shells it is 
possible to identify the particular formation from which the 
original limestone comes. Among the illustrations which accom¬ 
pany this paper will be found a photograph of a rock, which was 
originally pure limestone of the Vicksburg formation but is now 
changed, as shown by analysis, to a high grade phosphate. The 
shells and other fossils making up the limestone, which were 
originally calcareous, were subsequently phosphatized. Other¬ 
wise expressed, they have been petrified, phosphate being the 
petrifying agent. The collection of Dr. H. Bystra at Holder 
contains a piece of phosphate boulder, in which much larger 
shells are equally well preserved. While occasional phosphate 
boulders with fossils in a perfect condition of preservation are 
found as a rule the preservation of the fossils in the boulders is 
imperfect. It is probable, also, that in many boulders formed by 
replacement the fossils are entirely obliterated. 
PHOSPHATE BOULDERS FORMED BY PRECIPITATION. 
Many of the phosphate boulders are formed in part or entirely 
by precipitation of calcium phosphate from solution in water. 
This is probably the method of formation of the laminated 
boulders. 
It is probable that replacement and deposition from solution 
are both involved in the formation of many boulders. 
