“Bread-Crust” Volcanic Bombs

Abstract

IT is easy to identify the writer of the friendly notice of Dr. Tempest Anderson's “Volcanic Studies” (second series) in NATURE of April 18, but, notwithstanding his high authority, I must adhere to my explanation of “bread-crust” bombs (p. 42), viz. that the cracks are results, not of internal expansion, but of contraction. I had the opportunity of studying a large example near the crater of Vulcano, with others of smaller size in the Lipari Islands, and brought away a “hand-specimen,” now in the Sedgwick Museum. The former has a compact and rather glassy “rind” about an inch thick, with an interior full of small vesicles. The cracks vary much in size, and the walls of the shallower converge (are rifts, in the strict sense of the term); they appear to be analogous with the cracks in septaria, etc., and I do not see how the formation of numerous vesicles filled with steam is to increase the volume of the “core” within the “rind,” for the process is not comparable with one of effervescence. Simple contraction of the crust seems inadequate, since these cracks, so far as I have seen, do not occur in lumps which are homogeneous throughout.