650 



J. V. LEWIS — ORIGIN OF PILLOW LAVAS 



Geikie suggests that portions of the lava that were the last to solidify 

 were "forced into the crevices between the already solid pillows/*^ 



Where conditions suitable to their production exist, the spaces still 

 remaining unfilled offer favorable places for the wonderful variety of 

 beautifully crystallized minerals for which some of the localities are 

 noted. These include quartz, calcite, the zeolites, datolite, prehnite, 

 pectolite, epidote, and many other less abundant species. Secondary 

 processes, especially in lavas that become deeply buried, have transformed 

 the glassy crusts and interstitial fragments in many flows into green 

 chloritic mixtures, and in many regions corresponding changes have 

 taken place within the crystalline lava as well. These processes are well 

 illustrated by the spilites and the greenstones. 



7. RADIAL JOINTING 



Columnar jointing, when developed in pillow lavas, naturally forms 

 at right angles to the cooling surfaces, as in other columnar lavas ; hence 

 the radial arrangement so commonly seen in the spheroidal, ellipsoidal, 

 and pillow-like masses. In many examples this structure is developed in 

 great perfection, and in some localities it is emphasized by the deposi- 

 tion of calcite in the joint cracks, but as a rule it is most distinctly seen 

 in weathered sections. A shell-like concentric parting is found in some 

 pillow lavas, with a strongly developed zonal structure, and it is charac- 

 teristic of the tachylite crusts generally to scale off easily and break into 

 fragments. 



8. DEGREE OF VESIGVLARITT 



The individual masses become more or less vesicular according as 

 much or little gas is retained in solution by the liquid lava. In many 

 examples the outer crust is entirely free from vesicles, and concentric 

 layers or scattering lines of cavities or amygdules appear only in the 

 crystalline mass within, the most scoriaceous parts of all occurring in 

 the central portions of the pillows. This would be the normal result 

 where the liquid remains in equilibrium, holding itg volatile constituents 

 until they are released by the process of crystallization. G-lass, being a 

 supercooled liquid that has become rigid without crystallization, often 

 retains its volatile matter in solution, and in such case does not become 

 notably vesicular. T^Hiere there is supersaturation of volatile constituents 

 their rapid escape makes the whole mass spongy with vesicles, and their 

 continued separation from the liquid interior in advance of crystalliza- 



wi A, Geikie : The geology of central and western Fife and Kinross. Mem. Geol. Sur- 

 vey of Great Britain. Scotland, 1900, p. 72. 



