
ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 331 
In Australia, India, and Southern Africa, and in Virginia 
and North Carolina, U.S.A., workable coal occurs on higher 
geological horizons—namely, in late Palzozoic and early 
Mesozoic strata. Here and there, but only at wide intervals, 
seams of economic value are also encountered in the younger 
Mesozoic systems (Jurassic and Cretaceous). The Cainozoic 
strata in several regions yield lignite or brown coal, as in 
North Germany, Italy, and Washington, U.S.A. The mining 
engineer who may be on the outlook for coal will do his 
best, therefore, to discover fossils—the presence of which 
will determine the geological horizon of the strata. Should 
the fossils prove the strata to belong to an older period 
than the Carboniferous, workable coals are not likely to be 
met with. Hopes, however, may be encouraged should the 
rocks prove to be of Carboniferous or later age. In sucha 
case the engineer would endeavour to ascertain the general 
character of the strata. Should thick marine limestones be 
present in the series, and the accompanying shales and 
sandstones yield only brachiopods, cephalopods, and other 
types of marine life, and should few or no traces of land- 
plants occur, then there would be little probability of dis- 
covering workable coal-seams. Should the strata, on the 
other hand, consist of rapidly alternating beds of sandstone 
and black or dark coloured shale with occasional seams of 
clay, and layers of clay-ironstone—such a succession might 
be considered hopeful. The appearance here and there of 
thin limestones of marine origin would not necessarily be 
an adverse sign, for coals and limestones are well known to 
occur, now and again, in one and the same group of strata. 
But should the constantly recurring beds of shale contain 
plant-remains, often very well preserved, while the sand- 
stones showed streaks and thin lenticular layers of coaly 
matter, and the clays were charged with rootlets, our hopes 
of encountering coal would be greatly increased. It must 
be remembered, however, that the occurrence of black shales 
is not of itself a sure indication of the presence of coal in 
any series of strata. In almost all geological systems, particu- 
larly in some of the older Palzozoic formations, black shales 
may occur without the accompaniment of even the most 
exiguous seam of coal, Let us suppose, however, that our 
