224 ^^^^ American Geologist. October, i89s 
(March, 1897), Prof. W. A. Tilden stated, as the outcome of 
some work on quartz, feldspar, and the other constituents of 
granite, gneiss, gabbro, schist, basalt and other minerals, twen- 
ty altogether, from different horizons and widely distant lo- 
calities, that they all yielded gas in which hydrogen is the pre- 
ponderating element. Next to hydrogen the most al^undant 
is carbonic acid. And he further makes the important state- 
ment that the volume of gas given off by these rocks, and 
which comes entirely from the minute cavities within them,* 
ranges from 1.3 to 17.8 times the bulk of the rock; that is to 
say, that a cubic mile of stone wottld give out from one to 
seventeen cubic miles of gas. Considering these figures the 
problem begins to assume a new aspect and our next ques- 
tion is. How many of these cubic miles of rock have we at 
command? Because it is evident that if we only have miles 
enough we can get enough carbon-dioxide. 
At this point in the enquiry I was stopped for a while. 
How is it possible to ascertain the amount that has been worn 
off the surface of the crystalline rocks since geologic time 
began? I laid the subject aside for a time. But soon the 
thought occurred that the mass of the sedimentary rocks, with 
a few corrections, must equal the amount worn off the crys- 
tallines since the days when these latter composed the whole 
surface. But it is not easy to obtain even this datum and any 
result must be merely approximate. 
I may here be allowed to digress for a moment to explain. 
All geologists who accept the principle of cosmic evolution 
(and in the present day few can be found who reject it) are 
agreed that the earth has cooled and consolidated from an 
early liquid mass, consisting of slaggy, glassy and stony 
material resembling modern lavas. From this hard and 
intractable rock-mass all our sandstones, shales, clays and 
limestones have been slowly separated by the disintegrating 
*As a proof of this fact Prof. Tilden incidentally remarks: "The ^as 
is apparently wholly enclosed in cavities which are visible in thin sec- 
tions of the rock when viewed under the microscope, but as they are ex- 
tremely minute, very little gas is lost when the rock is reduced to a 
coarse powder, and a re'sult of experiment in one or two cases I find that 
practically the same amount of gas is evolved on heating the rock, 
whether it is used in small lumps or in powder." 
