ELLIPSOIDAL LAVAS IN GLACIER NATIONAL PARK 237 



So far as the Glacier National Park exposure of the Purcell lava 

 is concerned, the following criteria would seem to indicate the 

 bottom of a subaqueous flow : (1) discreteness of the basal spheroids 

 and their relative competence to resist mashing; (2) comparative 

 unevenness, with reference to the top; (3) the irregular displace- 

 ment of the underlying shale by the basal spheroids; and (4) the 

 presence of vesicles near the base of the individual flows. Criteria 

 indicating the top are: (1) the common ropy structure; (2) more 

 or less complete fusion of the individual spheroids; (3) comparative 

 evenness, with reference to the bottom; (4) silting up of hollows 

 in the top by strata whose laminae parallel those of the adjacent 

 strata; and (5) the absence of vesicles in the upper portions of 

 the individual flows. 



The flow under discussion covers an area hundreds of square 

 miles in extent, and while its extrusive character has been recog- 

 nized by various observers, the ellipsoidal structure has only 

 been found at the localities described. It may have been sub- 

 aqueous in places, subaerial in others (the Siyeh argillites are 

 abundantly ripple-marked and sun-cracked in places), but many 

 lines of evidence seem to prove its subaqueous character at the 

 locality described, and indicate that ellipsoidal structure is a com- 

 petent criterion of subaqueous extrusion. 



