ELLIPSOIDAL LAVAS IN THE GLACIER NATIONAL 

 PARK, MONTANA 1 



LANCASTER D. BURLING 

 Geological Survey, Ottawa, Canada 



The paper by Capps on "Some Ellipsoidal Lavas on Prince 

 William Sound, Alaska," 2 recalls to my mind a similar occurrence 

 which I visited in 1907. The locality is now so accessible and the 

 flow is so clearly subaqueous in character that a brief description 

 of it may be of interest. Its outcrop appears in the ridge (Shepard 

 Mountain) northeast of Flattop Mountain, Glacier National Park, 

 Montana, and the features described are in that portion of the bed 

 which overlooks the Shepard Glacier. The lava, to which the name 

 Purcell lava has been commonly applied, interrupted the sedi- 

 mentation of a flat-lying, greenish argillite which forms the upper- 

 most part of the Siyeh formation of the pre-Cambrian. This 

 argillite lies in normal position, and the portions above and below 

 the lava bed are macroscopically identical. 



The Purcell lava is approximately 150 feet thick on Shepard 

 Mountain and can be traced for miles to the southeast, north, and 

 northwest. It is composed of six or more successive flows, each 

 of uneven and more or less ropy surface, separated by small and 

 more or less local accumulations of shale. The lower 25 or 30 

 feet of the flow is composed of a conglomeration of dense, homo- 

 geneous, spheroidal masses averaging 1 to 2 feet in diameter. 

 They preserve their shape in the lower layers, being separated 

 from each other by chert or drusy cavities, and many individuals 

 have displaced considerable portions of the mud upon which they 

 were rolled or shoved, even to the extent of complete burial. The 

 bottom of the flow is therefore exceedingly irregular. Toward the 

 top of this bed the individual spheroids yield more or less to the 



1 Published by permission of the Deputy Minister of Mines. . 



2 Jour. Geol., XXIII (1915), 45-51. 



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