
ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 333 
fuel has resulted from the chemical alteration of vegetable 
matter while excluded from the action of the atmosphere. 
Coals vary much in character—even individual seams vary 
considerably, being more or less bituminous as the case 
may be. These differences are doubtless largely due to the 
nature of the original vegetable débris. Certain parts of the 
plants of Carboniferous times, for example, were more resinous 
than others, and when such enter largely into the formation of 
a seam we have usually a highly bituminous coal. The cones 
and spikes of the vascular cryptogams and the gymnosperms 
shed abundance of resinous spores and pollen, while some of 
the old coal-trees seem to have secreted resin—the cortical 
substance of certain types being traversed by tubes and canals 
which are believed to have been connected with this function. 
Leaves, woody matter, and bark constitute the major portion 
of most mineralised coal. 
The two views at present maintained as to the mode of formation of 
coal-seams may be very shortly described. 
(a) Growth in Situ.—This is thought to be the probable origin of 
coal-seams that retain a tolerably uniform thickness over extensive areas, 
and which rest on clay or shale containing abundant rootlets. Seams of 
this kind are supposed to have been formed much in the same way as 
mangrove- and cypress-swamps. The vegetation may have been 
developed partly on the low flat shores of estuaries and bays, and partly 
in the salt water itself, for marine shells now and again are found closely 
associated with such coal-seams. Many successive beds of coal may 
occur in a very thick consecutive series of sandstones, shales, clays, etc.— 
all these beds having been laid down in relatively shallow water. Such 
a succession would indicate accumulation during a prolonged period of 
subsidence—interrupted, perhaps, by longer or shorter pauses. Each 
coal with its underclay would in this view represent such a pause in 
the movement of subsidence. 
(b) Drifted Vegetable Débris.— Some coals cannot possibly have 
resulted from the growth of plants zz sztw. This is proved by the 
constant dovetailing and interosculation of such seams with shale and 
sandstone, and by the very irregular thickness of the coals themselves. 
Well-preserved ferns, leaves, branches, etc., may abound in the beds 
immediately underlying, and particularly in those overlying such coals. 
Now and again stems of trees in approximately upright positions occur 
in the associated sandstones. Their appearance is suggestive of flotage 
—for a long bare stem, crowned atop with an abundance of leafy 
branches, and weighted below with its thick but-end and roots, would 
tend to sink in a more or less upright position, swaying over, perhaps, 
in the direction followed by the transporting current. 
