CENTURY OF IOWA GEOLOGY 419 



Springer, another Iowa scientist. In direct continuation of this 

 paleontological cJicf d'ouvre other vohinies are now in press. 



Notwithstanding the fact that the work is first of all morpho- 

 logical in character from the foundation up, and the product of 

 inquiries more thoroughly grounded in biological philosophy than 

 any other research perhaps ever undertaken in this country, the 

 published results are of such high utility in stratigraphy, especially 

 in the broad Mississippi basin, that it may be truly said no other 

 one publication has ever furnished so valuable criteria for the pur- 

 poses of exact correlation of geological formations. 



Of all fossil remains of organisms none are more admirably 

 adapted to morphological study than those of the echinoderms. On 

 account of their abundance, their peculiarities in geographic and 

 geologic distribution, and their notable structure, the stalked feather- 

 stars, or stone Hies, are pre-eminent. With the skeletal parts com- 

 posed of regular plates, or ossicles, definitely grouped and fre- 

 quently highly sculptured, all structural changes are readily traced. 



The systematic arrangement of the crinoids as proposed by Wach- 

 smuth and Springer is one that will require but few material modi- 

 fications for a long time to come. Based, as it is, upon morpholog- 

 ical principles, with a completeness and wealth, of ontogenetic and 

 phylogenetic data that are rarely obtainable among fossil organisms, 

 the essential elements of classification are more firmly grounded 

 than perhaps in any other group. No attempt in recent years to- 

 wards a natural and orderly arrangement of a large and complex 

 assemblage of organic remains has been so signally successful. Nor 

 has the evolution of the various types in time and space been neg- 

 lected. 



Although the morphological and classificatory chapters of the 

 monograph on North American Crinoids appeal more directly to 

 paleontologists interested in the biological side of the subject, the 

 descriptive portions are of greatest practical value to stratigraphical 

 geologists. This portion of the volumes is a complete revision of 

 the different forms up to the time of publication. Every species is 

 fully and clearly described, compared with closely related forms, 

 beautifully illustrated, and referred to its, proper geological horizon. 

 All the forms are described anew from the most perfect materials 

 that could be found in all museums and private collections. 



DUALITY OF THE GLACIAL PERIOD 

 Admitting that Louis Agassiz's theory of continental glaciation to 

 be one of the most brilliant generalizations of modern science, it 



