620 J. V. LEWIS ORIGrSf OF PILLOW LAVAS 



nearly the entire flow. Between them is a softer greenish, locally schist- 

 ose material, the origin of which is not clear. The masses are commonly 

 somewhat flattened parallel to the dip of the flows, which, ia some exam- 

 ples at least, appears to be dne to the pressure of one ellipsoid on another 

 before they had completely solidified. Some of the masses are nnited by 

 small necks (plate 19, figure 1) ; some broken ones show a radial jointing. 



"The main volcanic actiritT seems to hare taken place near the shore line 

 of the Orca Sea. while forther out sediments were t>eing deposited, and finallr 

 sedimentation became the prominent phenomenon evai near the shore line.*' 



The characteristic feature of these rocks in the Ellamar district, where 

 they form thick accumulations of flows, is the ""abundance of beds in 

 i^ch the greenstone consists of ellipsoidal forms.-- 



"Ellipsoidal greenstones hare generally been considered to indicate that the 

 basic lava flows from which they were formed were poured out upon the 

 bottom of a body of water, and that the flowing and cooling of the laras 

 under water induced this unusual type of parting into ellipsoidal or spheroidal 

 forms. Many facts which can not be discussed here point to the conclusion 

 that these greenstones were extruded upon the sea bottom, and that the 

 deposition of the muds which formed the slates continued during the intM"- 

 missions betwe«i ttie s^iarate Aowsl'* 



Capps®' further defines the greenstone member of the Orca group as 

 consisting of "a number of lava flows which were intercalated between 

 the slate and graywacke beds, but which did not stop their deposition,** 

 and henc-e many beds of slate and graywacke were laid down between the 

 separate flows. The ellipsoids at the base of a flow are flattened at the 

 bottom, but rounded on the other sides, and the material between the 

 lower ellipsoids is the same as that of the underlying slates. The bottom 

 of a flow is flat and conformable with the stratification : but the top pre- 

 sents a ver}- different appearanc-e, ^"consisting of a succession of domes, 

 resembling the surface of a magnified c-obblestone pavement, the surfaces 

 of the ellipsoids representing the cobblestones.'' The mud next deposited 

 takes the shape of the irregular lava surfac-e. These surfaces show no 

 sign of weathering or erosion, and this is thought to add weight to the 

 conclusion that the lavas and interbedded sediments were deposited suc- 

 cessively under water. 



Maine. — ^In 1896 Smith*® described the dense amvgdaloidal diabases 



^ S. R. Capps and B. L. Johnson : Mineral deposits of the Ellamar district. BoIL U. S. 

 Geol. Survey. Xo. 542. lt»1.3. pp. 95. 96. 



® S. R. Capps : Report on the Ellamar district. Alaska. In mannscript. to which I am 

 permitted to refer by courtesy of the Director of the f. S. Geological Surrey. 



^ G. O. Smith : The geology of the Fox Islands, Maine. Dissertation, Johns Hopkins 

 Univ.. 1896, pp. 16-19. 



