SPHEROIDAL STUUCTUBE IN GENERAL 637 



Stromoberflache^' in spite of the fact that extensive fresh excavations 

 showed several flows composed wholly of these forms. Eeuning's pointed 

 question whether such a bed 70 meters in thickness can properly be con- 

 sidered the upper flow-surface of "Deckdiabas" is ignored. 



The Confusion of Hypotheses 



The foregoing summary of the principal available literature concern- 

 ing pillow lavas will serve to emphasize the aptness of Daly's characteriza- 

 tion of the problem : "The reason for the balling-up of the lava into rela- 

 tively small, completely separated pillows or ellipsoids is a physical prob- 

 lem of fascinating difficulty . . . but no one has yet made the matter 

 clear.'' ^^i 



From the study of considerably altered and metamorphic greenstones 

 possessing this structure, Rothpletz (1879) and Williams (1890) ascribed 

 the rounded forms, with their schistose chloritic interstitial matter, to the 

 movement of joint blocks on one another under the great pressure of 

 orogenic forces. The fresh and well preserved examples of the structure, 

 however, exclude the possibility of this explanation for the origin of the 

 pillows themselves. Credner (1876) and Russell (1878) attributed them 

 to brecciation in situ, and the latter regarded the occurrence at Glenside 

 Park (Feltville), New Jersey, as a friction-breccia developed during in- 

 trusion. 



Naumann (1834) thought the central Yogtland pillow greenstones 

 were conglomerates, and Winchell (1892, 1894, 1895) viewed the similar 

 greenstones of the Lake Superior region as agglomerates, produced by the 

 falling of bombs and a small amount of finer pyroclastic material into 

 the sea. But in 1899, while still maintaining his former conclusion, 

 Winchell conceded that their nature and origin were problematical. 



During the past 15 years all students of this structure have been sub- 

 stantially agreed in regarding it as essentially a flow phenomenon. Many 

 have thought it chiefly if not wholly characteristic of subaqueous flow, 

 and some recognize the possibility of either subaerial or subaqueous ori- 

 gin. When, however, definite processes are sought in their hypotheses — 

 specific explanation of how, for example, a flow of this character, covering 

 many square miles and having a thickness of scores or even hundreds of 

 feet, can be produced from liquid lava — all of them are exceedingly in- 

 definite. Hypotheses that scarcely agree in anything else are strikingly 

 alike in their pervasive vagueness — "the natural result," says Professor 



^ R. A. Daly ; Igneous Rocks and Their Origin. New York, 1914, p. 338. 



