112 



THE CRYSTAL FALLS IKON BEARING DISTRICT. 



TUK ELLIPSOIDAL STRUCTUKE IN THE METAUASALTS. 



Upon exaininiiig- the flat surfaces of many of the lavas one is immedi- 

 ateh' struck by their resemblance to a conglomerate formed of round 

 bowlders, all of the same kind of rock, lying in a matrix of very small 

 quantit}' and of A'ery different color. 



Fig. 7 is a sketch showing a portion of such a lava flow. 1 find that these 

 ellipsoidally parted rocks have been called "massive conglomerates," and the 

 blocks have been spoken of as "bombs" in the manuscript notes of some of 

 the men who have worked among them. The latter term was undoubtedly 

 due to the resemblance of the ellipsoids to the spindle-shaped pieces of lava 



fiG. 7.— Sketch of the surface of the 



utcrop of aD ellip.soi(lal basalt, showing the general character of the ellipsoids and 

 matrix. 



which one finds around the modern volcanoes. Ellipsoidal basalt is very 

 common throughout the Hemlock volcanic area. It is found most frequently 

 in isolated ledges. However, it is also associated with and grades into non- 

 ellipsoidal varieties. In one good exposui'e it is overlain by a fragmental 

 scoriaceous mass wliich separates it from another mass of similar ellipsoidal 

 basalt. While the scoriaceous portion may represent the brecciated surface 

 of a lava flow, it is not so considered, but is presumed to be a tuff deposited 

 upon the flow represented by the ellipsoidal basalt. According to this view 

 the ellipsoidal basalt is on the surface. In another exposure an ellipsoidal 

 basalt overlies a bed of water-deposited clastic rock. There is no passage 

 between the two kinds of rock. The contact between the two is an undu- 

 lating one, and is marked by a mass of schistose material about 2 inches 

 thick and similar to that which is between the ellipsoids. This particular 



