24 KEMP'S ORE DEPOSITS. 



formed, by the oxidation and combination with silica, of inner 

 metallic portions of the earth, for such is one of our most reason- 

 able explanations of volcanic phenomena, suggested alike by the 

 composition of basalts, by the high average specific gravity of the 

 globe, and by analogy with meteorites. 



1.03.04. As opposed to this conception, there are those who 

 would derive the metallic elements of ores from the ocean, in 

 which they have been dissolved from its earliest condensation. 

 Thus it is said that substantially all the metals are in solution in 

 sea water. From the sea they are separated by organic creatures, 

 it may be, through sulphurous precipitation, attendant on the de- 

 cay of their dead bodies. The accumulations of the remains 

 of organisms bring the metals into the sedimentary strata. Once 

 thus entombed, circulation may concentrate them in cavities. 

 When present in igneous rocks, the latter are regarded as derived 

 from fused sediments. If the metallic contents of sedimentary 

 rocks do not come from the ocean in this way, the igneous rocks 

 as outlined above are the only possible source. No special men- 

 tion is here made of the metamorphic rocks, because in their origi- 

 nal state they are referable to one or the other of the two remain- 

 ing classes. But it is not justifiable, in the absence of special 

 proof, to consider them altered sediments, any more than altered 

 igneous rocks, and it is doubtless true that the too generally and 

 easily admitted sedimentary origin for our gneisses and schists has 

 materially hindered the advance of our knowledge of them in the 

 last forty years. 



1.03.05. Microscopic study of the igneous rocks has shown 

 that, with few exceptions, the rock-making minerals separate from 

 a fused magma on cooling and crystallizing, in a quite definite 

 order. 1 Thus the first to form are certain oxides, magnetite, spec- 

 ular hematite, ilmenite, rarely chromite and picotite, a few sili- 

 cates, unimportant in this connection (zircon, titanite), and the 

 sulphides pyrite and pyrrhotite. Next after these metallic oxides, 

 etc., the heavy, dark-colored, basic silicates, olivine, biotite, au- 

 gite, and hornblende, are formed. All these minerals are character- 

 ized by high percentages of iron, magnesium, calcium, and alumi- 

 num. They are very generally provided with inclusions of the 

 first set. Following the bisilicates in the order of crystallization, 



1 H. Rosenbusch, "Ueber das Wesen der Kornigen und Porphyrischen 

 Structur bei Massengesteine," Neues Jahrbuch, 1882, ii., I. 



