76 The American Geologist. February, i899 
lithographs of the coal-period, perhaps in better condition 
than any other. 
6. Another method which I have experimented with in 
order to test pitch-coal laminae for masked anatomical struc- 
ture, is polishing. Blit while a finished polished surface may 
reveal nothing of this a subsequent rubbing on soft leather, 
until quite warm, will sometimes reveal decided indications of 
structures. The result of this frictional heat produced upon 
the leather (the sheep-skin covers of a quarto volume, in my 
case), seems to contract the denser and blacker areas, and to 
elevate the material now occupying the cells or tubulous parts: 
at any rate the result is an uneven surface, which, when viewed 
in a favorable light, shows differences or markings pertaining 
to original vegetable tissues. I submitted drawings made 
from a specimen thus treated (a bit of one of the black, brittle 
laminae out of the "Pittsburg" seam in Pa.) to Dr. David 
White of W^ashington. D. C. who showed them to Prof. F. H. 
Knowlton. and I am indebted to them for the remark that 
the markings could not be those referable to wood, but might 
possibly be tliallopJiytic. It is interesting to observe in con- 
nection with this method of finishing polished specimens of 
coal, that (in the case of bituminous coal at all events) where 
the above mentioned etched surface is produced. — all the bright 
lustrous, pitch-coal lines or laminae become depressed below 
the surface of the intervening or cementing-matrix material : 
the latter always standing out in relief as it appears in con- 
trast with the enclosed shining coal. I am myself not certain as 
to exactly how the irregular surface thus created comes about; 
possibly there is a shrinkage produced in one material and 
a swelling in the other; or the pitch-coal may remain /// statu 
quo, while the duller matrix swells. Anyhow the leather- 
rubbing process does not seem to remove any 'material, but 
merely acts upon the already smooth and even surface, 
through the medium of heat, so as to produce an uneven sur- 
face. What the temperature rises to when the rubbing is 
suf^cient I cannot say. but it is probably about iio° F. On 
cooling to normal temperature the efifect is )wt destroyed. 
Another and rather instructive side-light upon coal-forma- 
tion comes from the interiors of the so-called "sulphur balls" 
of the miner. Favorable specimens of these balls, which con- 
