99 



biti^men. Owing to the viscosity of such a continuous phase the 

 particles of mineral matter with which it is associated may be 

 regarded as a colloidal state, although they may be of dimensions 

 which would prevent their existence in such a state with a more 

 mobile liquid, such as water. Clay and finely divided silica present 

 such a relation to a highly viscous liquid, asphalt for instance, which 

 may be regarded as a colloidal one. Attention was attracted to the 

 subject in the course of a study of the native asphalt found in the 

 Pitch Lake in the Island of Trinidad, British West Indies. This 

 deposit is unique from a geophysical standpoint. It exists in a crater 

 6f an old mud spring on the West Coast of the island, and at a distance 

 of about half a mile from the Gulf of Paria. Its surface was originally 

 138 feet above sea level. Borings which have been recently made 

 show that the crude asphalt exists to a depth of more than 175 feet 

 at the centre of the deposit, which consists of a bowl-shaped mass 

 covering, originally, an area of 114 acres. Specimens taken at various 

 points on the surface and at different depths show that it originates 

 in an asphaltic petroleum, derived from oil sands occurring at con- 

 siderable depth below the lake, with which a paste of mineral matter 

 and water, originating in a mud spring, has become associated by 

 the churning action of the natural gas accompanying the petroleum, 

 on the release of the pressure to which it has been subjected as the 

 oil approaches the surface. The material formed in this way is of 

 highly uniform composition in all parts of the deposit, and consists 

 of an emulsion of bitumen with a paste of clay and fine sand, and 

 has the following composition : — 



Per Cent. 



Bitumen 39 



INIineral matter 27 



Water and gas, volatile at 100° C. - - - 29 

 Water of hydration of mineral matter - - 5 



100 



The water, which on melting the asphalt under certain conditions 

 can be separated therefrom, in a somewhat concentrated condition, 

 has been found to contain in solution large amounts of sodium chloride 

 and sulphate with a considerable amount of ammonium and ferrous 

 sulphates, together with borates and a readily recognisable percentage 

 of iodides. It also contains smaller amounts of potassium, calcium, 

 and magnesia salts. It is plainly of thermal origin. 



Refined Asphalt. 



As it occurs in the deposit it is known as crude asphalt. As such 

 it is submitted to a process of so-caUed refuiing at a temperature of 

 325° F., which removes the water and results in a material known 

 as refined asphalt, which has the following composition : — 



Per Cent. 



Bitumen - 57 



Mineral matter ------ 39 



Water of ^hydration of clay . . - . 4 



100 



G 2 



