IOWA ACADEMY OF SCIENCES. 108 



all been singularly indecisive in results. The extraneous 

 sources of error have not been guarded against. Far too 

 much has been assumed. Not only has the work been not 

 determinative, but in the published accounts the most 

 important point of all, that of the geological conditions, 

 has been scarcely ever touched upon. 



To be sure, problems of this kind are beset with many 

 difficulties. Many factors have to be taken into consider- 

 ation. There are theoretical conditions imposed that have 

 to be fully satisfied before actual examinatians can be of 

 much value. Singularly enough, existing critical data on 

 the subject have not come from those sources in which 

 special effort has been made to obtain the desired results. 

 They have been derived entirely in connection with micro- 

 scopical petography, incidentally, as it were. There are 

 strong logical reasons, as well as practical reasons, for 

 believing that in the case of the igneous rocks the ques- 

 tion can only be definitely disposed of through means of 

 the microscope. 



GENESIS OF CERTAIN CHERTS. 



BY CHARLES R. KEYES. 



As regards origin, the cherts which occur so abun- 

 dantly throughout the limestones of the lower carbonifer- 

 ous of the Mississippi valley have been the subject of much 

 speculation. The same problem has received attention 

 from every quarter of the globe. While the explanations 

 offered differ very much from one another they in general 

 agree in that they are regarded as formed contemporane- 

 ously with the geological formations in w^hich they occur. 

 The writings of Prestwich, Irving, Van Hise, Hull, Benard, 

 Hinde, Hardman, Hovey, and many others corroborate this 

 statement. 



