234 Native Bitumens and the Pitch Lake of Trinidad. (April, 
tion of common coal, that the greater number of its purest layers 
consist ofsthe flattened bark of the sigillariz and similar trees, 
just as any single flattened trunk imbedded in shale becomes a 
layer of pare coal. It also agrees with the fact that other layers 
of coal, and also the cannels and earthy coals, appear under the 
microscope to consist of finely comminuted particles, principally 
of epidermal tissues, not only of the fruits and spore-cases of 
plants, but also of their leaves and stems.” 
Every one, I think, must have observed, at some time, decaying 
logs, or better, stumps, of which little or nothing remains but a 
cylinder of bark, and this is apparently little altered. Dawson 
has found such hollow stumps in the coal formation, with abun- 
dant evidence that they had been the homes of animals, such as 
insects and reptiles. Such phenomena are the best illustrations 
of the superior resistance which this class of vegetable tissues 
offers to atmospheric action, a resistance undoubtedly due to the 
small proportion of oxygen which they contain; their composi- 
tion, as Dr. Hunt has pointed out, approaching closer to resins 
and fats than to wood, and, “like these substances, they repel 
water, with which they are not easily moistened.” 
We have now traced to their origin in the vegetable kingdom 
all of the coals, so far as known, and many of the true bitumens. 
The notion is rapidly gaining ground among geologists, however, 
that the bitumens, especially the lighter and more fluid forms, 
such as petroleum and naphtha, are largely of animal origin. 
This view, for the development of which we are mainly indebted 
to Dr. T. Sterry Hunt, is based upon the following general con- 
siderations: (1) Animal tissues, the average chemical composi- 
tion, but not the molecular structure, of which may be represented 
by the formula Ca, Has Ng O4, approaches even more nearly than 
epidermal vegetable tissues to the composition of bitumens. (2) 
Although, as a rule, eminently unstable compounds, subject, 
under ordinary circumstances, to rapid and complete decomposi- 
tion; yet we have good reason to believe that there are vast 
regions where the conditions are not only favorable for, but must 
necessitate, that slow and partial decay resulting in the formation 
of bituminous substances. The regions referred to are the depths 
of the ocean. Recent researches have shown, contrary to the old 
idea, that the deep sea holds an abundant fauna. All grades 
of animal life, from the highest to the lowest, have need 
