636 J. V. LEWIS ORIGIN OF PILLOW LAVAS 



mate relation with marine sediments as to suggest the possibility of intrusion. 

 An example of this kind is described by Fox and Teall from the greenstone 

 of the Lizard and Mullion Island, where intimate association with radiolarian 

 cherts suggested that the lava was intruded between the sheets of chert near 

 the surface of the sea bed upon which they were being deposited. This 

 intimate association with radiolarian cherts also found in the Arenig lavas 

 of Great Britain seems at present the only good indication of the probable 

 submarine origin of the lava. So far as the pahoehoe"® type of surface is 

 concerned, it appears to be equally characteristic of subaqueous and subaerial 

 extravasations. That constant and reliable minor differences exist between 

 subaerial and subaqueous lava surfaces is scarcely to be doubted, but at 

 present such differences appear to be unrecognized." 



In discussing the pillow or ellipsoidal basalts and the "spilitic suite" 

 of igneous rocks, Daly^^^ says : 



"In most cases their special students have concluded that they represent 

 subaqueous flows. The reason for the balling-up of the lava into relatively 

 small, completely separated pillows or ellipsoids is a physical problem of 

 fascinating difficulty ; the structure appears to be connected with the develop- 

 ment of the 'spheroidal state' at the contact of water and superheated basic 

 lava, but no one has yet made the matter clear." 



Daly rejects Dewey and Flett^s^^^ contention that "the pillow lavas are 

 members of a natural family of igneous rocks, the spilitic suite," and 

 continues : 



"In conclusion, the writer believes that the spilitic rocks are pneumatolytic 

 derivatives of normal basaltic magmas and that the modifying gas is chiefly 

 water of resurgent, not juvenile, origin." 



Yon Wolff^^® classes pillow structure with ropy pahoehoe, which owes 

 its features to slow viscous movement of a smooth-surfaced lava, and the 

 glassy surface coating of which is wrinkled and folded into various forms. 



"Fladenlava, Wulstenlava, Taulava, Pahoehoelava, Helluhraunlava, . . . 

 Diese Lava bewegt sich langsam wie eine zahe, plastische Masse ohne intensive 

 Gasabgabe, welche die erstarrte Oberflache auf reist. Dieselbe ist von einer 

 zahnen, glasglanzenden Haut bedeckt, die sich bei der Bewegung runzelt, zu 

 Falten, strick- oder tauformig gedrehten Fladen zusammenschiebt." 



Von Wolfl refers to the flow-surface features of the Upper Devonian 

 ''^Deckdiabas," and remarks that it sometimes shows pillowy (wulstig) 

 forms. Eeuning's "Kugeldiabas" ^^^ is characterized as "eine Art der 



^^ Including pillowy ; the author treats the two terms as synonyms. 

 ^'' R. A. Daly : Igneous Rocks and Their Origin. New York, 1914, pp. 338-340. 

 "8 H. Dewey and J. S. Flett : On some British pillow lavas and the rocks associated 

 with them. Geol. Mag., vol. viii, 1911, pp. 202-209, 241-248. 

 ^ F. V. Wolff : Der Vulkanismus. Stuttgart, 1914, pp. 372, 373. 

 1^0 E. Reuning : Neues Jahrb. fiir Min., etc., B. B., vol. xxiv, 1907, pp. 390-459. 



