30 PROCEEDINGS OF THE INDIANAPOLIS MEETING. 



dence of the gentle advances of the lake waters over the rough, wind-tossed and 

 water-sculptured surface of the plains, they possess an interest of their own as evi- 

 dence of a long period of emergence before the last submergence. The rivers had 

 time enough to cover the surface with their lines of erosion even more completely 

 than the surface is covered at the present time. The channels now occupied by rivers 

 show, here and there, marks of preexisting channels, and those which are still unoc- 

 cupied remain over to the credit of the older drainage system. 



All this series of events falls within Tertiary time. The older drainage system of 

 which I have spoken wrought upon Tertiary beds, and the erosion thus produced 

 makes the later Tertiary unconformable with the earlier. We have here the evidence 

 of cycles of emergence and submergence, of arid and humid epochs, of wind-swept 

 plains and ancient rivers, of structural forms invaded by agents of erosion and again 

 reconstructed — all within the limits of Tertiary history. In order to ravel completely 

 the tangled threads of this history it would be necessary to pass in review the events 

 accompanying the upheaval of the Rocky Mountains, perhaps also the physical history 

 of regions more remote. That does not, however, belong to this discussion. I have 

 merely aimed to decipher the geological record so far as to discover a probable cause 

 for the peculiar structural forms which have escaped destruction simply by reason of 

 their peculiarities. I have reached the conclusion that they are the result of sedi- 

 mentation upon a surface previously shaped by the action of the winds. In other 

 words, the lagoon type is a combination of the sedimentary and eolian types of con- 

 formation. 



The last paper an the programme was — 



ON THE RECOGNITION OF THE ANGLES OF CRYSTALS IN THIN SECTIONS. 



BY ALFRED C. LANE. 



The paper is published elsewhere in this volume. 



The Committee on Photographs presented a report of progress which was 

 accepted, and the committee was continued. 

 The Society then adjourned. 



