1887.] 



NATURAL SCIENCES OF PHILADELPHIA. 



319 



On the value of the faunal element in geological chronology. Some 

 geologists, and notably those who profess but little knowledge of 

 paleontology, have attempted to make light of the evidence which 

 the zoological record brings to bear upon the classification of rock- 

 masses. Indeed, even at the present time, in some of our surveys 

 only a left-handed assistance is asked by the geologist-in-charge 

 from the paleontologist, with the result known to all who are capa- 

 ble of distinguishing between work that is done and that which still 

 requires to be done. It is true that in many tracts, and in tracts 

 even of wide area, the relative sequence of rock formations can fre- 

 quently be traced without the aid of paleontology, and a map, more 

 or less perfect, constructed on the details thus brought together. 

 But soooner or later, if the comparative study of a region is neces- 

 sitated, reference must be had to the fossil remains, which alone serve 



