592 J. V. LEWIS ORIGIN OF PILLOW LAVAS 



Page 



Pillow lavas and spheroidal structure in general 634 



The confusion of hypotheses 637 



Formation of pahoehoe lava 639 



Formation of aa lava 641 



Development of spheroidal and pillow-like forms 644 



A theory of bulbous budding. 646 



General discussion 646 



1. Highly liquid lava 646 



2. Small continuous supply 646 



3. Bulbous budding 647 



4. Semi-molded, immobile forms 647 



5. Separation of individual pillows 649 



6. Interspheroidal cavities and breccia 649 



7. Radial jointing 650 



8. Degree of vesicularity 650 



9. Hollow pillows 651 



10. Extent of flow 651 



11. Contact with water 652 



12. Intrusive pillow lava 652 



Acknowledgments 653 



General Characters of Pillow Structure 



The structure here called pillow lava has been variously described as 

 cushion-like, sack-like, globular, spheroidal, ovoid, egg-shaped, ellipsoidal, 

 lenticular, and concretionary. Once regarded as rare and peculiar, but 

 now widely known in many countries, it has given rise to a great diversity 

 of opinion among geologists concerning its real nature, and hence to an 

 equal diversity of hypotheses as to the mode of its formation. Among 

 the numerous causes to which it has been attributed are included : sphe- 

 roidal weathering, spheroidal jointing, brecciation in situ, columnar joint- 

 ing, with subsequent movement of the columns on each other, concre- 

 tionary action, explosive eruption (bombs in agglomerate), normal flow 

 of lava on land, viscous flow and fracturing of stiff lava, lava flow under 

 water, intrusion into unconsolidated sediments, fracturing and partial 

 remelting of lava crusts, and rapid cooling and parting into separate 

 masses by the action of steam. 



Sir Archibald G-eikie- gives the following definition : 



"Pillow-structure (Ellipsoidal structure) — an arrangement in many ancient 

 and modern lavas where the rock before consolidating has separated into 

 globular or pillow-shaped blocks from a few inches to several yards in diam- 

 eter. The outer shell of these spheroids or ellipsoids is sometimes closer 



2 Textbook of Geology, London, 1903, vol. i, p. 136, 



