634 J. V. LEWIS ORIGIN OF PILLOW LAVAS 



Pillow Lava and spheroidal Stkuctuee in General 



In 1866 LyelP^^ believed globular structure to be recognizable only in 

 decomposing greenstone, basalt, and other trap rocks, but considered it 

 to be a development of an original structure previously invisible, in ac- 

 cordance with the views of Delesse,^^* which he summarizes as follows : 



"According to the theory of M. Delesse, the center of each spheroid has 

 been a center of crystallization around which the different minerals of the 

 rock arranged themselves symmetrically during the process of cooling. But 

 it was also, he says, a center of contraction produced by the same cooling. 

 The globular form, therefore, of such spheroids is the combined result of 

 crystallization and contraction." 



In 1869 Thomson^^^ stated that prismatic jointing is usually attributed 

 to one or both of the following principles : 1. Prismatic fracture by 

 shrinkage in cooling, like mud or starch in drying. 2. An assumed sphe- 

 roidal concretionary action of the lava or basalt in solidifying from the 

 molten state. Lyell, Daubeny, Jukes; and others are cited as having ad- 

 vocated the spheroidal concretionary theory, according to which both the 

 longitudinal and cross- joints were supposed to be different parts of the 

 surfaces of spheroids that have grown larger in solidifying till they have 

 met and squeezed themselves together so as to receive flattened faces in- 

 stead of a rounded form. This theory Thomson believed to be founded 

 on a total mistake. 



As late as 1876 Bonney^^® confuses supposedly original spheroidal 

 structure with that produced by weathering, as shown by his illustrations, 

 and ascribes its origin to contraction while cooling. 



In connection with Platan ia's description of the globular basalts of 

 Acireale, Johnston-Lavis^^^ makes the following observation : 



"A careful examination of the globular basalts shows that many are not 

 simple globes but rather pear-shaped masses with a narrow neck, which is 

 often absent, having been divided while still fluid. I have lately seen many 

 beautiful examples illustrating this mode of production in my rambles in 

 Iceland ; parts of Cape Reykjanes serve as a good example." 



From the association of spheroidal greenstones and diabases with radio- 



los Sir Charles Lj^eU : Elements of Geology, 1866, pp. 618, 619. 



104 A. Delesse : Sur les roches globuleuses. Mem. Soc. geol. de France. 2d ser., tome Iv, 

 1846. 



^°^ James Thomson : The .lolnted prismatic structure of the Giant's Caviseway and 

 other basaltic rocks. Belfast Naturalists' Field Club, 7th Ann. Rept., 1869-1870, pp. 

 28-34. 



106 T Q Bonney : On columnar, fissile, and spheroidal structure. Quar. Jour. Geol. 

 Soc. London, vol. 32, 1876, pp. 140-154. 



10" H. J. Johnson-Lavis : The South Italian Volcanoes. Naples, 1891, p. 43, footnote. 



