IOWA ACADEMY OF SCIENCES. 101 



fold, and the loss on ij^nition is very much greater. A 

 comparison of the water supplies of several cities shows 

 that the Sioux City supply is better than those of Brooklyn, 

 Boston, and Cincinnati and quite as good as those of 

 several other cities. 



IGNEOUS ROCKS OP THE CENTRAL CAUCASUS, 

 AND THE WORK OF LOEWINSON-LESSING. 



BY CHARLES R. KEYES. 



(Abstract.) 



In view of the widespread interest that the subject of 

 the differentiation of rock-magmas is exciting among geolo- 

 gists generally I am led at this time to call your attention 

 to one of the most recent, and at the same time one of the 

 most important contributions which has yet been made. 

 My notice will be brief, and will consist chiefly of an 

 exhibition of specimens of many of the most notable rock- 

 types. An explanation of some of the most significant 

 features will be given. Photographs of some of the most 

 characteristic rock-masses as they appear in the field will 

 be shown. These were obtained during a recent trip 

 through the Caucasus region in company with the Russian 

 investigator himself, guiding one of the excursions of 

 the Seventh International Geological Congress. 



Although the great work of the Russian petrographer, 

 F. Loewinson-Lessing, on the Eruptive Rocks of the Cen- 

 tral Caucasus, was issued more than two years ago, the 

 views advanced are only beginning to get into form 

 accessible to the majority of English students. The gen- 

 eral interest lies in the discussions of the subjects of rock- 

 classification and the differentiation of rock magmas. 



The classification proposed for the igneous rocks is 

 chemical. It is based primarily upon the degree of acidity 

 of the silicate minerals. Four great groups are thus 



