bhaler.] INTRODUCTION. 11) 
soil for grain tillage is measurably proportionate to the phosphatic con- 
tents. It seems almost certain, though not yet demonstrated, that the 
greater part of the phosphatic matter in the soil is in the state known 
asinsoluble, and that it only becomes in small part, year by year, soluble, 
or, in other words, fitted for assimilation by plants. Whenever the soil 
contains the quantity of lime which characterizes the better class of 
Kentucky soils it is supposed that even if soluble phosphatic manures 
are applied the superphosphate becomes again insoluble by taking 
up a molecule of lime. It is therefore an interesting question as to the 
means by which the lime phosphate enters the plants. It may be that 
the solution is effected through the action of the various humic acids 
of the soil or it may arise from some specific change which takes place 
at the contact of the soil with the roots. It is evident that this point 
requires precise determination, for on it will depend further experi- 
ments as to the methods of applying phosphatic manures, 
There is yet another point on which we need experiments. Many of 
our rock phosphates, especially those which are distinctly bedded, con- 
tain low percentages of phosphatic matter. Many of our lime phos- 
phates contain crystals of apatite and calcite so intermingled that it is 
not possible to separate them ; yet from these deposits it will be easy 
to produce a mixture of lime carbonate aud lime phosphate containing 
from 10 to 20 per cent, of phosphoric acid. The value of such material 
for manure has never been determined. If it can be used in a way which 
will give to the fields the full value for both the lime and the phosphorus 
it will open a way for an extensive production of cheap fertilizers. 
The foregoing considerations give the general results of the prelimi- 
nary inquiry into phosphatic manures of which Dr. Penrose's work forms 
a part. Before we go further into these studies I much desire to have 
the criticism and advice of others who have considered this subject. It 
is with this view that I have ventured to give in the foregoing pages an 
account of the aim of the inquiries I have in hand. The questions are 
at once chemical and geological, and demand much co operation for their 
solution. Much of the work of searching for the unknown phosphatic 
deposits of this country will necessarily have to be undertaken by local 
students of geology or by commercial explorers in search of such de- 
posits. Unfortunately, the unfamiliar aspect of the various forms of 
phosphatic deposits will make this task under any circumstances diffi- 
cult. There is no substance of equally wide diffusion among those of 
considerable commercial importance which, in the present state of pop- 
ular knowledge, so readily escapes detection as lime phosphate. It may 
be hoped that the following memoir may make it easier for explorers to 
recognize this class of deposits. 
My own as well as Dr. Penrose's acknowledgments are due to many 
persons who have given him aid in the prosecution of his work. To 
Prof. Charles U. Shepard, jr., of Charleston, S. C, Dr. Penrose is par- 
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