CHEMICAL CONCENTRATION 19 



acter were produced. In this manner ancient concentrates serve a pur- 

 pose analogous to that supplied by plant and animal fossils in determin- 

 ing the nature of former climatic oscillations. In attempting to apply 

 this principle, however, it should be remembered that during early geo- 

 logical eras the earth's interior heat may have played the role now 

 assigned to solar heat, and also that throughout the time embraced in 

 geological history the residual heat of lava beds may have produced 

 similar results. 



Of the concentrates resulting mainly from evaporation at the earth's 

 surface as in the zone of weathering, the next class in order of abundance 

 after the brines and salts contains the products resulting from the evapo- 

 ration of petroleum. 



Petroleum, when occurring in quantities of commercial importance, is, 

 as previously stated, a result of mechanical concentration, but when 

 evaporated under natural conditions yields gases, semi-solids, and solids. 

 The gases may be widely diffused in rocks as in the air, or concentrated, 

 according to the presence or absence of collecting or storage reservoirs. 

 The residues, either in a liquid or solid condition, left by the evapora- 

 tion of the more volatile portions of the original fluid, remain in the 

 cavities and fissures of greater or less size, or more or less completely 

 saturate porous rocks in the superficial portion of the earth's crust, or 

 are produced at the surface. The residues resulting from the evapora- 

 tion of petroleum constitute the naphthas, mineral tar, asphaltum, ozoke- 

 rite, grahamite, albertite, etcetera. 



In this connection, again, a knowledge of the mode of concentration 

 by which fractional parts of previously more complex or more hetero- 

 geneous material are segregated, aided by a knowledge of the nature and 

 kinds of receptacles necessary for the storage and preservation of the 

 concentrates produced, is an aid in the search for commercially valuable 

 deposits of the nature just considered. 



SUBLIMATION 



Analogous to the process of evaporation is the process of sublimation, 

 whereby matter is changed from a solid to a vaporous condition without 

 alteration in composition and redeposited as a solid when the tempera- 

 ture is suSiciently lowered. The process of sublimation as commonly 

 recognized, and so far as its geological action is concerned, requires a 

 high temperature in reference to the sun-derived heat of the earth's sur- 

 face, the source of which is the earth's interior. The heat is transferred 

 to the earth's surface as a part of the functions of volcanoes, and the sub- 

 stances sublimed may be considered as by-products of volcanic activity. 



