65 
1915-16.] The Origin of Oil-Shale. 
argillaceous, soft greenish and blue clays of great thickness and without 
any porous bands, though there are calcareous and ironstone nodules. 
The main manjak vein runs in a northerly direction at an angle of 
approximately 70°. It is lenticular in shape, and reaches a thickness of 
33 feet at about 180 feet below the surface. This is, I believe, the thickest 
vein of similar material ever recorded. There are also other veins in the 
neighbourhood, some running along the bedding for a considerable distance. 
The veins have not been worked deeper than 250 feet. The surrounding 
clays, though almost absolutely impervious to water, show on joint faces 
and slip planes spots and films of sticky oil, and the ironstone , nodules 
N S. 
CRETACEOUS STRATA 
Fig. 5. Diagrammatic Section through the San Fernando Manjak- Field, Trinidad. 
when broken open show similar films of oil and a slight impregnation due 
to the material being rather more porous than the clay. Firedamp is 
encountered in the workings, and has in one instance caused a serious 
explosion. 
It is evident from the nature of the strata and the size of the veins that 
enormous force must have been exerted to cause such intrusions. 
These phenomena can be paralleled in many oil-fields where similar 
conditions obtain, i.e. thick argillaceous deposits overlying oil-sands either 
near the crests or on the flanks of anticlinal structures. 
To recapitulate, the special points in connection with oil-fields to which 
it is wished to draw attention are : — 
(1) The stratigraphical relations of coal and lignite seams to oil- 
bearing strata. 
VOL. xxxvi. 
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