196 abstracts: hydrology 



further marked by laecolithic and semilaccolithic intrusions contem- 

 poraneous with the volcanic extrusions. The third period closely re- 

 sembled the first in that it witnessed the eruption of a single lava — -a 

 basalt — but it was characterized by the formation of a larger number 

 of cones and a less widespread distribution of the lava. 



The occurrence, megascopic and microscopic character, crystallinity, 

 together with the chemical and mineral composition of 23 types of 

 rocks ranging from a biotite rhyolite to an augite basalt are described 

 in detail and compared to certain "Reference types" established by 

 averaging the analyses of certain subrangs of the quantitative system 

 of classification. The average lavas of the composite cones are found 

 to have practically the same composition as the average lava of the 

 second period of eruption; it is thought that differentiation occurred in 

 a deep-seated zone and that the original composition of the magma was 

 probably basaltic; and it is concluded that the lavas may be considered 

 as representing all possible members of the igneous series which could 

 form under the magmatic conditions existing in the San Franciscan 

 region. Finally these lavas form a genetically related series of pro- 

 nounced continuity and are the differentiation products of an originally 

 homogeneous magma. These lavas were derived from the original 

 magma, or originated according to the laws of chemical solution. 



J. F. Hunter. 



HYDROLOGY. — Underground water resources of Iowa. W. H. Nor- 

 ton, W. S. Hendrixon, H. E. Simpson, O. E. Meinzer, and 

 others. U. S. Geological Survey Water-Supply Paper No. 293. 

 Pp. 994, with maps, section, and other illustrations. 1912. (Also 

 published as a report of the Iowa Geological Survey.) 

 This paper is a comprehensive and detailed treatise on the under- 

 ground waters of Iowa. Its chief contribution to geology consists of 

 numerous carefully interpreted sections of deep wells and their corre- 

 lation by the senior author. In the introductory chapter are outlined 

 methods of collecting and studying sample drillings and of interpreting 

 and correlating well sections. Paleozoic formations that outcrop in 

 the northeastern part of the State are traced by means of well sections 

 to the western part, where they are deeply buried under younger Paleo- 

 zoic, Cretaceous, and Pleistocene deposits. A contour map of the 

 upper surface of the St. Peter sandstone, one of the principal artesian 

 aquifers, shows this formation to be about 1000 feet below sea level at 

 Des Moines, and proba])ly 2000 feet below at its lowest point, a short 

 distance east of the southwestern corner of the State, toward which 



