306 
SILURIA. 
[Chap. XII. 
countries, or issues when the covering strata are pierced, proceeded from 
the ancient Carboniferous deposits which we have been considering, the 
explanation of the phenomenon would be comparatively easy ; for, whether 
the immense masses of matted vegetables of that period were solidified into 
coal and shale, or converted into the liquid substitute petroleum, the true 
source of that substance would be evident. In Shropshire, however, 
petroleum is seen to exude from fissures in old Cambrian rocks (see 
p. 27), in Caithness from ichthyolitic flagstones of the Old Bed Sand- 
stone (p. 258), and in North America it is said to issue chiefly from 
the Devonian formation, in a tract where comparatively few fossil vege- 
tables or animals are discovered in the rocks ; and in these cases the 
explanation is exceedingly difficult. The old belief of chemists was that 
bitumen can be derived only from vegetable or animal matter; and as 
neither of these in any appreciable quantity has been detected in the 
Cambrian rocks, we must in such case suppose that, though they have dis- 
appeared in the stony matrix, vast quantities of sea-weeds prevailed when 
these strata were in the condition of marine mud, and that the decompo- 
sition of these sea- weeds has produced the petroleum. But this view is 
not altogether satisfactory. 
Another theory is, that in some cases the presence of stone-oil is due 
to the former existence of Carboniferous strata over the oil-yielding area (as 
in parts of North America, for instance), and that the coal-fields were once 
extended over the ancient Cambrian rocks above mentioned, north and 
south of Shrewsbury, and, before and during their denudation, the bitu- 
minous contents of the coal-strata sank into the subjacent rocks through 
cracks and fissures, and occupied cavities within them. It is highly impro- 
bable, however, that this theory should be applicable to such cases as those 
where petroleum exudes from very ancient rocks, which rise into moun- 
tainous masses like the Longmynd, and which were unquestionably raised 
into dry lands before the coal-formations were accumulated. If, then, we 
retain the old chemical doctrine of the derivation of petroleum, we must 
revert to the theory that, as in all the marine deposits, however ancient, 
in which any traces of animal life exist there must also have been Algae, 
there may have been a sufficiency of them to account for the small amount 
of petroleum found in such rocks. 
In the younger deposits, as they begin to show an increase in vege- 
table and animal life, carbonaceous and bituminous ingredients abound 
more and more in the strata. That some of the shales of different epochs 
have been rendered bituminous chiefly through the abundance of animals 
entombed in them, may be surmised even in regard to the anthracite of 
the graptolitic schists of the South of Scotland, and to that which has been 
detected in the Silurian rocks of Cavan in Ireland (see p. 183). 
The presence of stone-oil in the Devonian rocks was probably due both 
to the conversion of masses of Sea- weed and to that of the numerous 
