75 The Ottawa Naturalist [August 



deep circular basin. By some it is supposed to owe its presence 

 to the action of a mud volcano, and the crater-like aspect of the 

 lake itself supports this view. By others the opinion is expressed 

 that it occupies the denuded crest of the La Brea anticline at this 

 point and that the crude petroleum has gradually flowed into the 

 denuded area, from the surrounding strata of oil-bearing sand- 

 stone, the volatile matter has been oxydized, and the remaining 

 asphalt now forms the lake. In the one case the denudation is 

 supposed to be caused by the agency of a gas explosion as with 

 ordinary mud volcanoes, in the other case it would probably be 

 caused by atmospheric agencies since it cannot V)e. supposed that 

 glacial action was ever experienced in these southern islands. 

 The origin of the pitch or asphalt is without doubt due to the 

 oxydation of the crude petroleum which flowed into the present 

 depression from the surrounding rocks. 



It was long supposed that the level of the lake was constant, 

 but when mining began on a large scale a careful series of levels 

 and other measurements was commenced. In this way it was 

 ascertained that in the 14 years during which mining has been 

 vigorouslv carried on the level of the surface has been lowered 

 seven feet, or at the rate of six inches per year. In this period it 

 is estimated that about 1,500,000 tons of the asijhalt has l^een 

 extracted and shiiJped. 



The surface is hard, and tlic asphalt is mined with an ordinary 

 pickaxe, the mineral breaking out readily with a sharp line of 

 fracture. It is loaded into tram^ cars and either sent to the ship- 

 ping point by a line of cable tram to the pier, or hauled along a 

 second tram line by mules to the ship]jing point or to the boiling 

 works where it is purified by the removal of the contained water 

 and of a certain amount of both organic and inorganic impurity. 

 The digging is made to a depth of one to two feet, when the trami 

 line is moved along the surface, but in a few weeks the depression 

 thus made is filled and the surface is again level. There appears 

 to be a certain slow movement going on which affects the greater 

 part of the mass, and lines of flowage are seen in the apparently 

 solid mineral as if the whole mass were in motion from the surface 

 downward. This movemxcnt is apparentlv due to convection 

 currents, which may be caused by the displacement of the whole 

 mass through mining or possibly to the still further and continued 

 inflow of semi-liquid pitch from the sides or bottom of the lake 

 basin. 



From the original lake basin im.m.ense quantities of the 

 asphalt have been discharged seaward to the shore v\here along 

 the beach it now extends for more than a mile. This beach 

 asphalt contains a somewhat larger percentage of impurity than 



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