63 
1915-16.] The Origin of Oil-Shale. 
impurities, but to many different analyses having been made by different 
observers, and also to the fact that samples taken from different parts of 
the same vein often show considerable differences. 
But for the fact that albertite and wurtzelite are distinguished from 
the rest by the very large percentages insoluble in carbon-disulphide, the 
series shows a fairly complete gradation, the percentages of malthenes or 
petrolenes (i.e. soluble in petroleum spirit) decreasing as the specific gravity 
increases. The part of the bitumen insoluble in petroleum spirit is called 
the asphaltenes. 
These “ native bitumens ” occur in veins almost invariably vertical or 
very highly inclined, though they send off minor branch veins along joint 
or bedding plains in any direction. Almost all the phenomena of igneous 
intrusion are simulated: sometimes there is a regular columnar jointing at 
right angles to the walls of the vein, and sometimes there are selvages with 
columnar jointing, while the centre of the vein, obviously a later intrusion, 
is characterised by conchoidal fracture. In such cases the conchoidal variety 
is always richer in malthenes or petrolenes and has a lower melting-point 
than the selvages. The material is truly intrusive, and has consolidated 
without reaching the surface. Weathered specimens taken near the surface 
are less soluble in petroleum spirit than specimens taken from greater depth ; 
and in fact, though it is evident that the material must have reached its 
present position in a liquid or semi-liquid state, the process of inspissation 
is still in progress. In one vein in Marbella Mine, Trinidad, a soft variety 
of manjak exuded slowly from the centre of a vein ; this material could be 
fractured by bending it sharply and quickly, but if allowed sufficient time it 
flowed slowly, adapting itself to every irregularity of the surface upon which 
it was placed. The malthene percentage was as high as 56 in this case. 
1. Barbados . — In a typical Barbados manjak-field the relations of the 
solid bitumen to petroleum are admirably illustrated. The structure is anti- 
clinal, but complicated by sharp minor folding in the lower strata (Scotland 
series) exposed. These strata are unconformably overlaid by the Oceanic 
beds, also thrown into anticlinal form, though much less pronounced, and the 
Oceanic beds are in turn unconformably overlaid by the Coral Limestone, 
which is horizontal. It is with the Scotland series alone that we are con- 
cerned. It is obviously in the petroliferous stage, every bed when un- 
weathered having at least a distinct odour of petroleum. The upper beds 
are chiefly impervious clays, but sandstone beds become more frequent in 
the lower horizons. It is through these strata that the manjak is intruded 
in somewhat irregular veins, which vary considerably in thickness and give 
