SPHENOIDAL STRUCTURE IN GENERAL 635 



larian cherts in some of the earlier known localities, TealP^^ concluded 

 that the combination is probably characteristic of marine, and possibly of 

 deep-sea, lavas. 



Sir Archibald Geikie is also of the opinion that pillow lavas in general 

 have a submarine origin, as expressed in the following : 



"Some basic lavas on flowing into water or into a watery silt have assumed 

 a remarkable spheroidal, sack-like, or pillow-like structure, the spheroids being 

 sometimes pressed into shapes like piles of sacks." "^ 



"They belong to the time when the lava was still in movement and when 

 it separated into globular portions, perhaps by flowing into water or muddy 

 sediment." "» 



"The origin of these rounded blocks has been ascribed to the sudden dis- 

 ruption and chilling of lava that has flowed into a lake, river, or the sea." ^ 



Kemp^^^ suggests a mode of origin when he defines ^^spheroidaF' as a 

 ^^descriptive term applied to igneous rocks that break up on cooling into 

 spheroidal masses analogous to basaltic columns.'^ 



Harker,^^^ in a discussion of the depth at which igneous intrusions 

 occur, says: 



"Under submarine conditions where the pressure of a column of water is 

 added to the weight of the rocks, sills may be intruded close to the sea floor. 

 Probably some are forced in along sediments actually in process of deposition 

 in deep water, the distinction between intrusion and extrusion being in such 

 circumstances of little significance. Here belong many of the rocks distin- 

 guished by what Sir A. Geikie has styled 'pillow structure.' " 



Iddings^^* merely suggests that pillow structure is a flow phenomenon, 

 without consideration of its immediate cause : 



"Occasionally lava in flowing separates into lumps or small masses that 

 assume rounded forms and when solidified in a closely pressed aggregation 

 have the appearance called pillow structure." 



Grabau^^^ includes pillowy structure among the characteristic surface 

 features of basic lavas. He says : 



"While the careful study of these structures in the cases cited has led the 

 observers to the conclusion that such features are reliable as indications of 

 surface flows, yet there are cases in which these surfaces have such an inti- 



^°® J, J. H. TeaU : On greenstones associated with radiolarian cherts. Trans. Roy. 

 Geol. Soc. Cornwall, vol. xi, 1894, pp. 560-565. 



^^ The Ancient Volcanoes of Great Britain. London, 1897. vol. i, p. 25. 



110 Textbook of Geology, 4th ed. London, 1903, vol. 1, p. 306. 



111 Textbook, vol. ii, p. 760. 



112 J. F. Kemp: A Handbook of Rocks, 3d ed. New York, 1904, p. 224. 



us A. Harker : The Natural History of Igneous Rocks. New York, 1909, p. 64. 



11* J. P. Iddings : Igneous Rocks. New York, 1909, vol. 1, p. 300. 



^ A. W. Grabau : Principles of Stratigraphy. New York, 1913, pp. 311-317. 



