126 
Thirty years ago it was considered that the soft or cherry 
coals were chiefly composed of the remains of large plants 
such as Sigillaria, Lepidodendron, &c., while caking coals 
were formed of plants of a lesser size and much bark, for it 
had then been observed and since confirmed, that the out- 
sides of Sigillaria and other fossil trees, sometimes reaching 
to two or three inches in thickness, were chiefly composed 
of bright soft coal showing little traces of structure. 
In the great lawsuit which was tried at Edinburgh more 
than twenty years since as to the nature of Boghead coal, 
much evidence was given as to its structure, some witnesses 
findiug in it scarcely anything else than the remains of 
vegetables, whilst others found only^a stray portion of scala- 
riform structure or a macrospore. Both Boghead and the 
other brown cannels of Scotland are now generally consi- 
dered to afford but little evidence of vegetable tissues under 
the microscope, although numerous remains of Sigillaria 
and other common coal plants can be seen by the naked eye 
in them. Notwitstanding that they are so rich in volatile 
matter, and far exceeding other coals in their yield of 
paraffine and paraffine oils, they contain from 25 to 30 per 
cent of mineral matter in the form of ash. This circum- 
stance has induced certain scientific men to class them as 
shales rather than coals, notwithstanding that they have the 
specific gravity of coals. 
Some years since, when describing several fossil cones 
affording both kinds of spores, he expressed an opinion that 
the yellow matter seen in the vesicles of Boghead coal was 
nothing but microspores composed of paraffine or a similar 
hydrocarbon, and that they were driven off by heat in the 
form of a yellow vapour, leaving nothing behind but a 
spongy mass of earthy and carbonaceous matter. The evi- 
dence that caused him to come to this conclusion was 
that the microspores contained in the upper sporangia of 
Lepidostrohus Harcourtii had all the appearance of crude 
