78 H. C. COOKE 



The methods of determining the positions of the upper surfaces 

 of the lava flows may be summarized as follows: 



A. In a single flow 



1. From changes in pillow structure with depth 



2. From changes of grain with depth 



B. At contact of two flows 



1. From observation of a coarse-grained, non-ellipsoidal 

 base resting on an ellipsoidal upper surface 



2. From observation of a coarse-grained base resting on a 

 fine grained upper surface 



The latter two methods are much the most useful. 



[To be continued] 



