ELLIPSOIDAL LAVAS ON PRINCE WILLIAM SOUND 51 



stones. Upon this surface later deposits of mud were laid down, 

 the bottom of the mud bed taking the shape of the irregular lava 

 surface. The lava flows show no signs of weathering or erosion 

 upon their surface, and the succeeding mud beds must have been 

 laid down soon after the lavas cooled. 



Summary. — -The greenstones near Ellamar, Alaska, comprise a 

 thick series of lava flows, most of the flows consisting of ellipsoidal 

 lavas, and many of them displaying this character in a striking way. 

 At frequently recurring intervals in the greenstone series there are 

 interbeded slates and graywackes, and rarely some limestone, the 

 sedimentary beds being in every case structurally conformable 

 with the overlying and underlying lavas. The surfaces of the 

 flows show no evidence of erosion or weathering, the succeeding 

 sediments having been laid down upon the fresh, unaltered lava 

 surface. If the greenstones are subaqueous flows, as is believed 

 to be the case, then they were merely interruptions during the 

 deposition of ordinary clastic sediments. 



If, on the other hand, the lavas were not subaqueous flows, and 

 were extruded upon land, it is necessary to postulate an elevation of 

 the surface above sea level for each extrusion of lava and an imme- 

 diate resubmergence for the deposition of the sedimentary beds. In 

 view of the many recurring sedimentary beds throughout the 

 greenstone series, this latter assumption seems most unlikely. The 

 absence of evidence of erosion or weathering on the surface of the 

 lava flows also supports the conclusion that they solidified below 

 sea level, as does the presence in the lava cracks of thin films 

 of mud which must have been plastic at the time they were injected. 



The difference in appearance of the flat bottom of an ellipsoidal 

 flow, with its abundance of mud-filled cracks, from the uneven upper 

 surface of the same flow, and the consequent unevenness of the 

 bottom of the succeeding sedimentary bed often made it possible 

 to determine which surface of a steeply tilted bed was originally 

 the upper surface, and was a valuable aid in working out the struc- 

 ture of the beds. 



