228 DR JAMES GEIKIE ON 
a vegetable structure. These two kinds of coal alternate in one and the same 
seam (Plate XIII. figs. 5 and 6)—sometimes a bed of glance coal being streaked 
with lamin of dull slaty coal, at other times a seam of slaty coal showing 
many thin lines of glance coal. This last, indeed, appears to be most usually the 
case, as the slaty coal is the commoner variety of the two. JOHNSTRUP states 
that every lenticular mass of glance coal represents the flattened trunk of a 
tree, in which can be seen the annual rings of growth. This, I do not doubt, 
may be true of the thicker layers, but it does not seem to be the case with 
the finer streaks and threads; at all events I could see no trace in these of 
flattened stems. But our examination was necessarily imperfect and in- 
complete. It seemed to me, however, that the alternating layers of bright 
and dull coal spoke to the gradual accumulation in water of different kinds of 
vegetable matter or of different parts of the same plants, and that the coal 
was analogous to what is sometimes seen in our Scottish coal-fields, where 
thin layers of gas-coal, black-band ironstone, and common coal alternate and 
interosculate in one and the same seam. 
The comparative composition of the two kinds of coal is shown in the 
following analyses, which are taken from JoHNSTRUP’S paper :— 
GLANCE COAL. 
1p IP 
Organic, . é ‘ ; 85°3 83'1 
Inorganic (Ash), ; : : 2:5 2°5 
Hygroscopice Water, . ; 12:2 14:4 
COMMON OR SLATE COAL. 
Good Coal. Bad Coal. 
if iT E Ts 
Organic, '! + 4 78:0 73°4 _ 65:0 60°6 
Inorganic (Ash), ‘ 10°7 9:2 16:2 29°3 
Hygroscopic Water, . 11:3 17°4 18'8 10°1 
JouNSTRUP tells us that some of the coals are extremely ashy, containing as 
much as 51 and even 74 per cent. of inorganic material. This latter, however, 
is rather a carbonaceous clay than a coal. In open-air sections it is not 
difficult to trace the passage from coal into shale—an appearance which, taken 
in connection with the general aspect of the beds, is strongly suggestive of the 
aqueous formation of the coal-seams. I saw no trace of a true underclay, and 
nothing resembling rootlets. Indeed I was rather surprised at the apparent 
scarcity of plant-remains in the shales. Dark vegetable impressions were 
observed, but there was nothing among these which could have been 
specifically determined. The shales and clays associated with the coals 
generally contain thin lines and layers of glance coal and dull commoi coal, 
and now and then small nodules of coarse grey clay-ironstone make their 
