T. 0. Bosworth — Outlines of Oilfield Geology. 57 



Oily Clays. — The exposed clays of a petroliferous formation do not 

 so readily lose their hydrocarbons, and often they are found to smell 

 of oil quite strongly, even where exposed at the surface. 



Burnt Clays. — Occasionally, as in Trinidad and California, there 

 occur beds of clay-shale, sometimes 10 feet or more in thickness, which 

 have been naturally burned (at depth) into hard red rock like brick 

 by the combustion of the bituminous material contained in them. 



Oil Seepages. — Oil exuding from outcropping oil-sands, or coming 

 up faults and joint-cracks, frequently seeps through the surface soil, 

 forming pools which are sometimes of considerable size. 



Likewise films of oil are often seen on the surface of water in 

 swamps, wells, and springs. The surface of the sea is sometimes 

 similarly overspread for miles. 



Pitch. — Pitch is one of the commonest indications of oil. Extensive 

 deposits occur in Mexico, California, Venezuela, Trinidad, West 

 Africa, Borneo, Russia, etc. The composition of the pitch depends on 

 that of the oil it is derived from, and also varies according to the 

 extent to which evaporation and oxidation hare advanced. Many 

 oil-sands near the surface contain sticky pitch, and spreads of pitch 

 mixed with sand often cover the outcrops and occupy the bottoms of 

 ravines into which they have flowed. Generally the pitch appears 

 on the surface as numerous cones, isolated or in groups. Usually the 

 cones are only a few feet or less in height, but in some places there 

 are pitch hills of 50 feet or more. Single cones are symmetrical, and 

 on the outside may be hard and dry, but at the apex is a little crater 

 from which flow takes place. When cut open it is seen that the 

 cone is built upon the surface soil above some crevice, up which soft 

 sticky pitch and thick heavy oil is slowly ascending through the 

 ground, and thence up the central pipe of the cone to the apex. 



In country covered with thick vegetation the outcrops of oil-sands 

 can be traced by the occurrence of pitch and the approximate course 

 of an anticline may be followed for miles. Pitch also occurs at 

 submarine outcrops, for soft fresh pats of pitch are washed up ashore 

 abundantly on the coast of petroliferous country. The extensive 

 deposits of pitch at present on the earth's surface bear witness to the 

 immense quantities of petroleum which have escaped into the 

 atmosphere in recent times. 



The best known individual and uninterrupted pitch spread in the 

 world is the Trinidad Pitch Lake with an area of about a quarter of 

 a square mile, but there are more extensive areas covered with pitch 

 in Venezuela. Estimating the total contents of the Pitch Lake at ten 

 million tons, it follows that some forty million tons of oil must have 

 evaporated at that one site alone. 



Bitumen in Fissures. — Fault fissures, joint-cracks, and cavities 

 among rocks above or associated with petroliferous strata, are 

 sometimes filled with bitumen derived from the oil. The bitumens 

 derived from asphaltic oils differ from the pitch which is formed 

 from the same oil at the surface, being hard, black, lustrous substances 

 which break with conchoidal fracture. One of the best known of the 

 many different kinds is manjak, which is mined in Trinidad and 

 Barbadoes. 



