Chap. XII.] 
COAL AND PETEOLEUM. 
305 
Godwin- Austen, in a memoir in which he delineated the north-western 
coal-area of Europe (Quart. Journ. Geol. Soc. vol. xii. pp. 38 &c, pi. 1). 
The later researches of Mr. H. C. Sorby, directed from another point of 
view, indicated the trend and character of the marine currents of the coal- 
bearing sea-margin. This author, to whom we are indebted for so many 
good observations in physical geology, has arrived, by independent re- 
searches, at the conclusion that the seas of the Coal-period (in the North 
of England) were enclosed and shallow, with no very fixed or determinate 
lines of drift or current. In some valuable notes with which he favoured 
me*, he shows that during the deposition of the older slate-rocks of 
Westmoreland and the Lake District, or the Lower Silurian time, the cur- 
rents came chiefly from the east, and that in the Upper Silurian period 
they proceeded from the N.N.W. These facts, when further developed, 
must throw great light on the physical geography of early times. He fur- 
ther indicates that during the period of the Carboniferous Limestone and 
Millstone-grit the sea was open to the west. 
These regular currents, however, as he shows, cease to be conspicuous 
when we rise into the Coal-measures in the same districts, the general cur- 
rent having then become almost extinct, and a confused set of tidal and 
wind drifts having supervened, which indicate a very considerable altera- 
tion in the outlines of the region. These changes point, first, to a diminu- 
tion of the current-flow in the period of the Millstone-grit, then to a far 
more limited extension of the water during the Coal-period, ending in a 
fresh set of currents during the Permian era, among elongated shoals. 
With such data before us, we are, perhaps, warranted in believing that 
a theory of the formation of coal which should embrace as its chief element 
a widely extended series of shallow and partially enclosed seas, fringed with 
swampy forests of water -loving plants, subject to frequent submersion, 
accords well with observed phenomena. 
In geological researches we frequently meet with results that cannot be 
fully explained by one modus operandi only; for nature has evidently 
worked out phenomena apparently similar through distinct processes ; and 
of this no better indication can be given than that coal may well have 
been formed out of vegetable matter by the above-mentioned different 
processes f . 
Bituminous Shale and Petroleum. — In closing this Chapter on the old 
Carboniferous deposits, I may be excused for saying a few words upon the 
recent discovery of the outflow of vast quantities of petroleum or stone- 
oil in North America, and of the distillation of the same substance from 
the bituminous shale of British Coal-fields, particularly those of Scotland. 
If all the stone - oil which exudes from the crust of the earth in various 
* See also his papers on this subject in the searches on the nature and character of Coal and 
Edinb. New Phil. Journ. vol. iii. p. 112, iv. p. 317, its mode of accumulation, will be found in his 
v. p. 275, vii. p 226. memoirs,' On the Structures of Coal,' Quart. Journ. 
t Various interesting conclusions arrived at by Geol. Soc. vol. xvi. pp. 268 &e. ; ' Air-breathers of 
Principal Dawson, from his long-continued re- the Coal-Period,' Montreal, 1863, p. 18, &c. 
X 
