GEOLOGY: E. BLACKWELDER 
491 
divides here and there, the element really progresses through a series of 
subordinate cycles all having a common beginning and capable of being 
brought eventually to a common end. 
Omitting the rare meteorites, we find the primary source of phos- 
phorus in magma (molten lava). Almost all magmatic rocks, whether 
they solidify in the interior of the earth or are poured out upon its sur- 
face, contain this element. According to Clarke,^ the average igneous 
rock of the world contains 0.29% of P2O5. The proportion is somewhat 
greater in the basic rocks, such as gabbro and the peridotites; and in 
the peripheral portions of certain magnetite ore bodies, which are be- 
lieved to be merely ultrabasic segregations from a less basic magma, 
the quantity of P2O5 may exceed 10%. In these rocks the phosphorus 
almost invariably appears in the form of the mineral apatite, a com- 
pound of tricalcium phosphate with calcium fluoride or chloride. 
The hot solutions and vapors emitted by subterranean bodies of 
magma, especially during the progress of crystallization, permeate the 
adjacent rocks, and, through the crystallization of their constituents 
upon the walls of fissures, are believed to form the veins known as 
^pegmatites.' In rare instances pegmatites contain noteworthy quan- 
tities of apatite — generally in large crystals. Some of the Norwegian 
apatite deposits are probably of this origin. 
There is another type of phosphatic veins which is even rarer than the 
pegmatitic variety, although locally in central Spain there are impor- 
tant deposits of this kind. These are the fibrous veins consisting not of 
apatite, but of the minerals staffelite or dahllite — hydrous calcium car- 
bo-phosphates^ containing about 38% of P2O5. These veins traverse 
quartzite, slate and limestone, but have a suggestive association with 
intrusions of granite. While their origin is not known, it may be sug- 
gested that they will eventually prove to have crystallized from mag- 
matic solutions at a considerable distance from the parent intrusion and 
at a comparatively low temperature. Certain even rarer primary de- 
posits containing phosphorus minerals may be neglected in this brief 
sketch. 
At and near the surface of the earth, and under favorable climatic 
conditions, rocks of all kinds are subject to chemical decomposition. 
One of the chief agents of this process is ground water containing car- 
bonic acid and other solvent substances. In this weak solution apa- 
tite and the hydrous phosphates such as dahllite dissolve somewhat 
readily, as compared with most other common minerals, although much 
less rapidly than calcite (CaCOs). 
The dissolved portion circulates through the rocks and is disposed of 
