ELLIPSOIDAL LAVAS ON PRINCE WILLIAM SOUND 49 



lying sediments. The presence of this sedimentary material in the 

 cracks of the fractured ellipsoids can be satisfactorily explained 

 only by assuming that the bed of mud upon which the lava was 

 extruded was at that time soft and plastic, and that upon the cooling 

 of the lavas, and their cracking from shrinkage as they chilled, the 

 weight of the lava bed forced the soft mud from below up into the 

 cracks of the lava as fast as they were opened. In places these 



Fig. 4. — Steeply dipping ellipsoidal lava, on right, succeeded by conglomerate and 

 graywacke. The uneven lower surface of the conglomerate conforms to the top of the 

 underlying lava flow. 



mud-filled cracks extend several feet into the flow, and where best 

 developed give the rock a mosaic appearance, the fragments of 

 greenstone, often only a few inches in diameter, being surrounded 

 by a black layer of baked shale. The mud fillings of the cracks 

 vary in width from the thickness of a knife blade to several inches. 

 At one locality the openings between the ellipsoids were found to 

 be filled with limestone. 



In those places where thin ellipsoidal greenstone beds were 

 adjoined both above and below by sedimentary beds, it was found 

 that the two surfaces of the flow presented different appearances 

 (Figs. 4 and 5). The under surface in each case was flat, the ellip- 

 soids of the lower layer being flattened at the base, though presenting 



