4:76d Scientific Intelligence. 



rocks which have been formed since the beginning of the glacial 

 epoch. As thus denned they will probably include some rocks 

 which have been called Pliocene. 



The theory of the fundamental or principal classification of 

 these rocks which has actually obtained in the world differs ma- 

 terially from that of the classification of all other clastic forma- 

 tions ; as they are priinarly classed by agencies and conditions 

 of genesis. 



Progress has been made in classifying the glacial deposits 

 chronologically, and in classifying some of the lacustrine and 

 marine pleistocene deposits chronologically by correlating them 

 with the glacial epochs, or by a study of their fossils ; but it is 

 worthy of emphasis that the fundamental classification of all the 

 rocks of this period is one based on genesis. 



The series of geological events that constitute the history of 

 the glacial epoch in North America are exceedingly varied and 

 interesting. The great ice sheet that advanced from polar re- 

 gions into middle latitudes, and then retreated and again ad- 

 vanced and retreated, was a dominant geological agency, and 

 about its history all other phenomena are grouped. These phe- 

 nomena are found in the records of a great number of lakes, many 

 of which have disappeared, and in marine dejDosits which mark 

 the sites of old shores and which are intimately related to the 

 glacial deposits. In the Mississippi Valley the deposits of loess 

 extend nearly to the Gulf of Mexico, and these formations are 

 probably in part of glacial origin, and they have many intimate 

 relations with the deposits wholly glacial. The deformations of 

 the surface by diastrophic agencies through the glacial period 

 are many and profound, and their history is recorded in a variety 

 of base-level formations. When these facts are all recorded a his- 

 tory of the glacial period can be constructed which will be of 

 profound importance as an integral chapter in the history of the 

 earth, the principles of which will be extended to all the chapters 

 of the volume. In northern latitudes glacial phenomena prevail. 

 In southern latitudes many other phenomena can be correlated in 

 time with glacial facts, because glacial action was projected far 

 southward along mountain ranges and about isolated mountains, 

 and because the glacial sediments of the Atlantic coast extend 

 far southward and their histories are interwoven with the history 

 of the sediments derived from more southern rivers and shores ; 

 and finally, because glacier-prepared sediment was carried into 

 the valley of the Mississippi and spread far toward the Gulf. 



The classes usually recognized in practice by geologists of this 

 country are as follows : 



1. Formations of fluviatile origin, otherwise called " flood-plain 

 deposits," or alluvial deposits. 



2. Formations of lacustrine origin, or lake deposits. 



3. Formations of oceanic origin, or marine deposits along shore. 



4. Formations due to the agency of torrents, a large part of 

 which have been called alluvial fans or alluvial cones. 



