STRUCTURAL FEATURES OF THE PRE-C AM BRIAN 



77 



Ransome, Russell, and M. E. Wilson, proved so difficult and 

 uncertain of application that it was not used. That flattening is 

 greater on the under side than on the upper was clear in most cases 

 only when the position of the upper side had 'been previously 

 determined by other means. 



Grain. — A progressive increase in grain takes place in passing 

 from the top of the flow toward its base. At the top the lava is 



Fig. 2. — The curve to the left shows the change of grain in depth of a non- 

 eUipsoidal andesite flow. The curve to the right is that for an intrusive of the same, 

 thickness and composition. The maximum grain of the latter will of course vary, and 

 may be much larger than represented. 



almost or quite glassy, and the grain gradually increases in coarse- 

 ness with depth till the maximum of approximately i mm. (in 

 andesite) is attained a few inches from the bottom, where a narrow 

 chilled edge is found. In the pillow lavas the fine, glassy grain 

 persists throughout the whole thickness of the ellipsoidal zone. 

 The curve in Fig. 2 shows the change in grain with depth of a non- 

 ellipsoidal flow. For comparison the type of curve for an intrusive 

 of the same composition and thickness is introduced. 



It is obvious that if a sufficiently wide section can be obtained 

 across the outcrop of a flow to show this change in grain the position 

 of the top will be indicated. This, however, is only occasionally 

 found possible in practice. The method is more apphcable to 

 the basaltic flows than the andesites, as their maximum grain is 

 apt to be larger. 



