384 REVIEWS 
During the Pleistocene period a thick mantle of glacial deposits was 
spread over the entire area, which has been eroded far enough to 
uncover the rocks here and there. 
Clements’s description of the western part of the district treats of 
the surface features, the economic resources and the petrographical 
character of the various formations, especial attention being paid to the 
volcanic rocks. ‘The great abundance of volcanic breccias and tuffs 
indicates the probable existence in Huronian time of a volcanic cone 
in this region, but the possible lucation of its vent has not been dis- 
covered. A small part of the igneous rocks are acid, their area being 
too small to map on the scale of publication. They include rhyolite- 
porphyries and aporhyolite-porphyries and breccia of the latter. The 
great part of the volcanics are metabasalts and breccias of the same. 
An interesting development of ellipsoidal structure is noted. The 
pre-Cambrian intrusive rocks include granites and rhyolite-poryhyry, 
metadolerite, meta-basalt and picrite-porphvry, besides a series con- 
sidered to be closely connected genetically ranging from granite, 
tonalite and quartz-mica-diorite through diorite, gabbro, and norite to 
peridotite. The diorite is closely related to monzonite. 
In the second part of the monograph Smyth discusses at length the 
effect of buried magnetic ores on the magnetic dip needle, describes 
its use and the results of careful observations in locating the iron-bear- 
ing deposits. He also describes the different formations structurally 
and petrographically. The same is done by Bayley for the Sturgeon 
River Tongue. J: 3P ae 
The Geography of Chicago and its Environs. By Roun D. 
SALISBURY and WiLLIamM C. ALDEN. Bulletin No. 1 of the 
Geographic Society of Chicago, published by the Society. 
Chicago, 1899. 64 pp. 
This pamphlet is a model essay on local geography written in an 
interesting style and illustrated in an attractive and instructive manner. 
From the maps and descriptions it is learned that Chicago is situated 
on a plain which stretches from Winnetka, sixteen miles north, to 
Dyer, about twenty-eight miles south of Chicago, and sweeps eastward 
around the southern end of Lake Michigan. This plain is narrower at 
its extremities and has a maximum width of fifteen miles in about the 
latitude of Chicago; it is imited on the east and northeast by Lake 
