236 LANCASTER D. BURLING 



pressure of their fellows, and they unite to form an upper surface 

 of moderate unevenness. The upper part of the entire flow is 

 composed of a bed about 20 feet thick, which, though massive in 

 character, is very porous. Vesicles are common near the base of 

 several of the individual flows in the lower portion of the lava. 



On Mount Grinnell, 10 miles to the southeast, Finlay 1 gives the 

 thickness of the lava bed as 42 feet, but does not mention the ellip- 

 soidal masses which Daly later describes from the same locality. 2 

 Finlay records the discovery of five genetically connected dikes 

 on Flattop Mountain close to the localities where the ellipsoids 

 are present. Elsewhere, though the lavas reached the surface 

 through numerous widely scattered dikes, ellipsoidal structure has 

 not been recorded. This period of igneous activity has been 

 described 3 as having genetically connected extrusive and intrusive 

 phases, and it is interesting to note that strata, upon whose upper 

 subaqueous surface lava was being extruded, should have been 

 able, at a depth of only 600 feet, to accommodate themselves to the 

 essentially contemporaneous intercalation, along single planes, of 

 intrusive sills scores of square miles in extent. 



This flow seems to afford an excellent opportunity for determin- 

 ing the value of certain criteria for distinguishing (1) subaerial 

 from subaqueous flows, and (2) the top from the bottom of sub- 

 aqueous flows. Here the normal attitude of the flow and its includ- 

 ing sediments is unquestionable, and the bottom of the bed in which 

 the ellipsoidal structure is developed is far more uneven than the 

 top, an observation which lessens the importance of one of the 

 criteria advanced by Capps. Furthermore, the silting up of cracks 

 in the surface of the flow would seem more natural than the upward 

 penetration, into cracks several feet in height, of mud sufficiently 

 resistant to flatten the bases of individual ellipsoids. That the 

 latter is true for the Prince William Sound locality 4 merely illus- 

 trates the difficulty of obtaining competent and unconflicting 

 criteria. 



1 Bull. Gcol. Soc. America, XIII (191 2), 350. 



2 Memoir Gcol. Survey Canada, No. 38, Part I (1912), p. 217. 



3 Daly, ibid., pp. 218-20. 



4 Capps, Jour. Gcol., XXIII (1915), 49. 



