1915-16.] The Origin of Oil-Shale. 61 
contain the inspissated remnants of a saturation with oil. A notable case 
in point is the fine-grained rock of San Fernando Hill in Trinidad. This 
deposit is known as argiline, and, though the oil-bearing Tertiaries have 
long been removed from above it, at a depth of some 8 or 10 feet from the 
surface a slight impregnation with dried-up petroleum is very obvious, 
while more porous sandstones of the same (Cretaceous) series in the 
neighbourhood show not a sign of oil. Similar evidence both in Trinidad 
and Venezuela may have caused the Cretaceous formation in these countries 
to be considered as a primary oil-bearing series. 
When oil-seepages are noticed at the surface, or oil-shows struck in a 
well in such very slightly porous but impregnated strata, it is remarkable 
that the oil is invariably lighter in gravity than that of the main supply 
of oil in the district. This is not so much due to the protection against 
inspissation that the less porous rock affords, as to the natural filtration to 
which any oil that has migrated through argillaceous or slightly porous 
rocks is subjected. This filtration removes wholly or in part the heavier 
fractions of the petroleum, and thus by analysis it is usually a simple matter 
to detect a filtered oil. The proportions of the other constituents in the 
petroleum remain relatively to each other much the same, but the “ residue ” 
and the heavier lubricating fractions, together with the solid paraffin (if the 
original or primary oil should contain that product), are greatly reduced, if 
not actually eliminated. The heavier molecules, as seems only natural, are 
less active in migration through capillary interstices, or perhaps more 
correctly have a much lower surface tension, and hence are outstripped by 
the lighter molecules. 
This phenomenon of filtration of petroleum is very familiar to oilmen. 
A good instance of it is the water- white oil of Kala Deribid in Persia, which 
seeps slowly from a very compact and fine-grained shaly rock. Its original 
source is probably in sandstones of a higher horizon, which have many of 
the characteristics of oil-rocks, but now contain no trace of having been 
such beyond a little sulphur staining. 
Perhaps, however, the most remarkable case of filtration is furnished by 
the Calgary field, where commercial results have unfortunately as yet failed 
to equal the results of scientific interest. A series of wells drilled in some- 
what complicated structure within a distance across the strike of about half 
a mile have furnished a series of oils varying from one with 20 per cent, or 
30 per cent, of residue to an oil with about 2| per cent, of residue, 6 per 
cent, of lubricating oil, and 72 per cent, of petrol. The oil becomes lighter 
the further east it is struck and the higher in the geological series. The 
oils all give clear evidence of a common origin, but on their migratory 
