DISTRIBUTION BRITISH ISLES 605 



"as if it had rolled down a declivity and become partially cooled during its 

 progress, and then consolidated into the rock which it now constitutes; in 

 fact, much like the ends of bales of cloth piled one on another." 



Eeference was also made to this structure by Fox^^ in 1902. 



The flow consists of masses 2 to 8 feet in diameter, many of which have 

 large central cavities ranging up to 2 feet. The pillows are highly 

 vesicular (plate 20, figure 1), having only about one-half the density of 

 the solid lava, and are associated with purely marine strata. Hence the 

 authors believe the lava to be a submarine eruption consisting of separate 

 spheroidal individuals that rolled on cushions of steam and so light that 

 the whole sheet probably moved almost like a liquid. 



"It seems therefore that the lava was in a true spheroidal state, each large 

 drop ejected swelling up independently and forming a pillow more or less 

 surrounded by escaping steam, so that the flowing mass on the sea bed formed 

 a mobile sheet of rolling spheres, seldom touching one another till they cooled. 

 As soon as the steam condensed, however, water would be sucked into the 

 vesicles and the pillows would settle down." 



The pillows are partially molded on each other and calcite fills the vesicles 

 and interstitial spaces. 



Geikie and Strahan*^ have described a series of tuffs and basic flows 

 (originally olivine basalts) interbedded with fossil -bearing Carboniferous 

 limestones at Weston-super-Mare, Somerset. One sheet of dull green 

 vesicular rock, 12 to 14 feet thick, shows marked ellipsoidal structure, 

 with masses 2 to 8 feet in diameter, and a rugged scoriaceous upper sur- 

 face, the irregularities of which are filled and overlain by fossiliferous 

 limestone. Morgan and Reynolds*^ found the rock locally variolitic, and 

 Boulton*^ concluded that the limestone between the pillows at Spring 

 Cove, at any rate, came from the beds below. From a study of the asso- 

 ciated limestones and their fossils, Strahan*^ concluded that the lavas 

 were parts of a submarine volcanic eruption which merely interrupted the 

 deposition of the fossiliferous limestone. Some of the more hardy species 

 began to struggle back immediately afterward and their remains are 



*i Howard Fox : Some coast sections in the parisli of St. Minver. Trans. Roy. Geol. 

 Soc. Cornwall, vol. xii, 1902, p. 670. 



*2 A. Geikie and A. Strahan : Volcanic group in the Carboniferous limestone of North 

 Somerset. Mem. Geol. Survey of Great Britain. Summary of Progress for 1898, pp. 104- 

 111. See also Geikie's Textbook of Geology, 4th ed., 1903, vol. ii, p. 757. 



^-'^ C. L. Morgan and S. H. Reynolds : The igneous rocks associated with the Carbonifer- 

 ous limestone of the Bristol district. Quar. Jour. Geol. Soc. London, vol. 60, 1904, pp. 

 137-157. 



** W. S. Boulton : On the igneous rocks at Spring Cove, near Weston-super-Mare, 

 Quar. Jour. Geol. Soc. London, vol. 60, 1904, pp. 158-169. 



*s A. Strahan : Geogr. Jour., vol. xxxix, 1912, p. 130. 



