36 
Structure of a 'peculiar Combustible Mineral, 
always black and do not alter in colour, whereas the latter 
become brown, the same as other parts of the mineral. 
3. Characters exhibited by sections under the Microscope. — 
There appear to be two principal varieties of this mineral, 
one of a yellowish-brown colour, the other nearly black, these 
differences, however, are chiefly dependent upon the position 
the particular fragment selected for section occupied in the 
block. When the first variety is reduced sufficiently thin to 
be transparent, which can be done without much difficulty, it 
will be seen to consist of a tolerably uniform, yellow mass, 
whilst the darker variety is either of a rich brown, or of a 
pale-yellow colour, minutely spotted with black granules. 
When the first or yellow variety is examined with a power of 
50 or 100 diameters, it exhibits an appearance of being made 
up of a mass of transparent rounded particles or spherules of 
a rich yellow or amber colour, varying in size from the ^^^'^ jjth 
to the Yooth of an inch (as shown in Plate III., fig. 1), whilst 
the darker variety (fig. 2) is composed of two essential ele- 
ments, one in the form of the transparent rounded particles, 
the other minutely granular, but black and opaque, and 
occupying the spaces between the yellow particles. In the 
first variety of this mineral, or that which is of a yellowish- 
brown colour in section, the yellow particles above alluded 
to are so very abundant, that they appear almost to make up 
the entire mass, whilst the dark granular element is small in 
quantity. In the second, or dark variety, the strictly granular 
opaque element is much more abundant ; it sometimes occurs 
in large patches, having none of the yellow particles with it, 
but more frequently it is found in the form of a coating to 
the particles themselves. When the yellow particles are of 
large size, they always exhibit more or less of a radiated 
structure internally: this appearance, which is well represented 
in figs. 1 and 2, very much resembles that of a radiated frac- 
ture, or a species of crystallization. I shall now, for the sake 
of distinction, call all these yellow particles, or spherules, the 
bitumenoid or combustible portion of the substance, and the 
dark, granular part, I shall consider as the strictly mineral, or 
earthy ingredient. 
In some specimens there is a tolerable regularity in the size 
of the yellow particles, and in the disposition of the black 
mineral ingredient around them, so much so that an unprac- 
tised eye might, at first sight, consider its structure to be cel- 
lular : that such mistakes have actually been made you will 
very soon have an opportunity of learning. 
Having told you what is the usual structure of the substance 
in question, I must beg you to understand that it matters little 
