Shaler.] 408 [Dec. 7, 



Mr. S. H. Scudder gave an account of the means adopted by the 

 butterflies of the genus Basilarchia for the perpetuation of the spe- 

 cies. 



General Meeting, Dec. 7, 1887. 



Vice-President, Dr. G. L. Goodale, in the chair. 



Prof. N. S. Shaler read the following paper : 



ORIGIN OF THE DIVISIONS BETWEEN THE LAYERS 

 OF STRATIFIED ROCKS. 



BY N. S. SHALER. 

 {Published by permission of the Director of the U. S. Geol. Survey.) 



It is a well-known fact that the greater part of our stratified 

 rocks are divided into distinct layers or beds of varying thick- 

 ness and generally of somewhat diverse composition. I am not 

 aware, however, that the origin of these divisions has ever been 

 made a subject of extended inquiry. The very generality of the 

 phenomena has served to hide its importance. To a great extent 

 our geologists have accepted the existence of stratified planes as 

 something inherent in the nature of sedimentary deposits. Where 

 they have sought an explanation of their existence, they have gen- 

 erally found it in presumed geographic changes though they rarely 

 have been able to indicate the character of the change which has 

 led to the institution of the divisions between the beds. 



As a preparation for the study of the alterations which have oc- 

 curred in the coast line of North America I have found it necessary 

 to consider the effect of geographic change in producing altera- 

 tions in the character of sedimentary deposits. It was obviously 

 necessary to consider the evidence afforded by the successive layers 

 of our sedimentary formations and to determine how far they could 

 be taken as evidence of geographic change. Although these con- 

 siderations are but a fragment of a considerable inquiry, they seem 

 to me to have a sufficient importance to justify their presentation 

 apart from the matter with which they will finally be associated. 



The only part of this subject connected with stratification which 



