Review of Recent Geological Literature. 377 
ness of its central belt from Labrador and the south part of Hudson 
bay westward to the Pacific varied apparently from one to two miles, 
and the area which it covered was approximately 4,000,000 square miles* 
Stratigraphy of the Carboniferous in central Ioxoa. By Chakles R. 
Keyes. Bulletin, G. S. A., vol. ii, pp. 277-292, with two plates; March 
5, 1891. A section extending 65 miles, from Harvey in Marion county 
west-northwest along the Des Moines river to the city of Des Moines 
and thence westward along the Raccoon river to De Soto, is here de- 
lineated ; and descriptions of the strata are given for the ten localities 
included within this distance. There is in general a very gentle dip 
toward the southwest. Mr. Keyes finds from careful measurement of 
the various members of the Lower Coal Measures, that they originally 
were more than 700 feet thick ; but they have suffered much erosion, 
and probably nowhere in this district present now so much as half of this 
thickness. They are chiefly shales, with infrequent and thin layers of 
sandstone. One exceptional bed, however, called the Redrock sand- 
stone, has a length of at least twenty miles and a width of six or seven 
miles, and attains a maximum thickness of more than 150 feet. Only a 
few very thin bands of limestone, mostly nodular and shaly, are found; 
but, though seldom exceeding ten or twelve inches in thickness, they 
constitute the most persistent horizons of the series, being recognizable 
over wide areas. 
The coal seams vary from a few inches to seven or eight feet in thick- 
ness, the average of those at present worked being between four and 
five feet. They occur as lenticular masses, from a few hundred yards 
to several miles in diameter ; and along the line of this section more 
then twenty coal-bearing horizons have been found, some of them having 
several of these lens-shaped layers of greater or less extent. The fauna 
of the section embraces about 50 genera and more than 150 species. 
Minute molluscan shells occur in vast numbers. 
Evidence of important oscillations of level during the deposition of 
the Lower Carboniferous formations and the Coal Measures is shown by 
unconformaties, some of which were reported by White more than 
twenty years ago. The most remarkable instance is the erosion of the 
Redrock sandstone in deep gorges and ravines, which afterward became 
filled with a coal deposit and shales. 
Anintroductlonto the study of petrology: The igneous rocks. By Fred- 
erick H. Hatch, Pii. D., F. G. S., 43 illustrations, J2mo., 128 pp., New 
York, Macmillan & Co., 1891. 
This is a handy compend of the characters of igneous rocks, as well 
as of the minerals of which they are composed. Its use presupposes a 
knowledge of all the customary methods of determination whether they 
be chemical or microscopic. It has chapters on their structure, chem- 
ical composition and alteration, as well as on their classification and dis- 
tribution in the British Isles. The work does not afford extensive refer- 
ences to literature, but it is evidently based on a wide acquaintance 
with the best authorities. It is adapted to beginners. 
