REVIEWS 643 
tain Associated Rocks,” by S. W. Beyer; ‘The Artesian Wells of Iowa,” 
by W. H. Norton, and the ‘Relations of the Wisconsin and Kansan 
Drift Sheets in Central Iowa, and Related Phenomena,” by H. Foster 
Bain. 
Mr. Leonard describes the formations which embrace or are con- 
tiguous to the lead and zinc deposits, the mode of occurrence of these 
deposits, the association of the minerals and the particular forms of the 
ores. ‘To these he adds special descriptions of the mines and a dis- 
cussion of the origin of the deposits and the general methods of 
working them. He makes an important contribution to the general 
relationship of the ores in showing that in the Dubuque district zinc 
occurs in the higher horizons of the Galena limestone associated with 
the lead. This appears to require a modification of the generalization 
previously reached in Wisconsin and northwestern Illinois to the effect 
that the zinc usually occurs in lower horizons than the lead. ‘The 
additional data appear to indicate that in their original deposition in 
the strata the zinc and lead were immediately associated with each other, 
and that their distribution in the crevices as the result of secondary 
action has been dependent upon the conditions of precipitation which 
were not uniform in all districts. Mr. Leonard regards the Archean 
rocks as the original source of the lead and zinc, having been derived 
thence by surface decomposition and carried into the Silurian sea, from 
which in turn they were precipitated along with the gathering limestone. 
The precipitating agency he thinks was chiefly organic. He discusses 
the different theories of the localization of the metallic deposits, and 
concludes that on the whole Chamberlin’s theory of oceanic currents 
offers the most plausible explanation. He regards the crevices as 
chiefly due to flexures of the strata aided by solution. He holds to 
the view that the minerals were carried into the crevices by lateral 
secretion from the surrounding limestones. 
The rocks which Mr. Beyer finds associated with the Sioux quartz- 
ites embrace a series of slates and some olivine diabases. The slates 
he finds to conformably overlie the quartzite and to be somewhat inter- 
bedded with or graduated into the upper quartzitic layers. He regards 
the slates as an upward extension of the quartzite formation. In 
respect to the thickness of the quartzite formation he favors the lower 
estimate of Todd (1500 feet) rather than the higher estimate of Irving 
(3000 to 4000 feet), but regards both estimates as doubtful. He con- 
firms the view of Irving that the quartzites were formed from siliceous 
