Correspondence. 6 1 
Beginning at page 96 in Cliap. 11, we find a valuable discussion of 
•"rock pressure of gas.'' Orton rejects the theory that this pressure is 
due to expansive force of the gas; also the theory that it is due to the 
weight of the superincumbent rocks; and thinks the true cause is 
hydrostatic pressure of subterranean waters, the same cause which pro- 
duces artesian wells. 
Chap. Ill, on the Trenton limestone will set prospectors everywhere 
to inquiring, "How far must we go to reach the Trenton.'" But even if 
they reach it the search will generally be in vain, for Orton shows con- 
clusively, by numerous analyses, that the Trenton is a gas or oil rock 
only when it has the composition of dolomite. 
Chap. IV on the Berea grit is of greater interest to the geologist than 
to the oil operator, since the latter does not find much encouragement 
from this horizon. The extent and continuity of the Berea, as shown 
'On the map p. 313, are truly surprising for so thin a stratum. 
"The Ohio Shale as a Source of Gas" is the subject of Chap. V. This 
stratum is probably the richest in the state, the amount of oil per square 
mile having been estimated at 10,000,000 bbls., notwithstanding it is 
practically of little value as an oil and gas rock. Low-pressure gas is 
obtained from it in many places, but no great wells. 
Chap. IX on measurement of gas wells, by Prof. T. W. Robinson is a 
valuable contribution to knowledge. 
Chap., XIII, "Natural and Artificial Cements" by Prof. N. W. Lord is 
interesting and valuable, equally to the scientist and the practical man. 
Chap. XV, by Edward Orton, brings out much new matter in regard to 
the production of lime, an industry which has been well nigh revolu- 
tionized by the introduction of natural gas. 
Many other portions of this work equally worthy of commendation 
■must be passed without inention for lack of space. 
A careful persual of this volume will convince every one that the 
royal opportvmities afforded to the geologist by the immense number of 
wells put down in search for gas and oil since the opening of the Find- 
Jay field, have been wisely used, and the interests of our science are 
there in able hands. 
CORRESPONDENCE. 
The Huronian of Canada. On page 238 of the American Geologist 
l(vol .1, No. 4,) Mr. S. A. Miller refers to Alexander Murray and the " Hu- 
ronian series," and says: " If he had read Emmon's Taconic System it is 
difficult to conceive why he should have hesitated in referring the rocks 
to that system;" and Mr. Miller further says: "The word Huronian is 
therefore a synonym for Taconic." 
I think Mr. Miller would scarcelv have made such a statement had he 
