292 O. Pouleft Scrope—On " Blochj" Rock Surfaces. 



and Loire, near Le Puy, in Central France ; the great lava-field at 

 the foot of the Peak of Teneriffe, within the vast encircling 'cirque' 

 described by Prof. Piazzi Smyth as a chaos of tumbled blocks, more 

 or less cuboidal, and varying in size from that of a house downwards. 

 Such, too, seems to be the character of the surface of the great lava- 

 bed of California, which was the last refuge of Captain Jack and his 

 tribe. 



Now what is the cause of this peculiar structure ? Shrinkage dur- 

 ing consolidation is the usual, and no dou.bt the correct, explanation. 

 But how happens it that while some lavas on exposure to the open 

 air are fissured and broken up into blocks in this extreme degree, 

 others present a smooth and almost glossy exterior, wrinkled into 

 cable-like folds, while others exhibit rude and jagged scoriform pro- 

 jections which have been compared to the waves of a storm-tossed sea 

 frozen at the moment of its wildest disturbance ? It is remarkable 

 that lavas poured out from the same volcano at successive intervals 

 sometimes present these distinct varieties of superficial structure, as 

 for instance the several Vesuvian lava currents of 1871 and 1872, 

 described by Prof. Palmieri (Account of the Eruption of March, 1872, 

 Mallet's translation, p. 103), the former as breaking on exposure to 

 the air into rude scoriform fragments (which he consequently calls 

 ' fragmentary lava') ; the latter coating itself with a uniform solid 

 skin, more or less wrinkled, which he calls 'lava with united surface.' 

 It is noteworthy that these two lavas differ also in mineral character 

 — the former being rich in pyroxene, the latter in leucite. 



In my work on Volcanos I have referred these differences in the 

 forms assumed by lava surfaces on exposure to the atmosphere to 

 corresponding difi'erences in the grain or size of the crystals or 

 granular particles that compose the lava at the moment of its out- 

 flow ; the finest grained lavas approaching to the state of glassy 

 fusion, being highly viscid and comporting themselves on exposure 

 like melted glass or obsidian ; the coarser-grained being yet suffi- 

 ciently liquefied to admit of the formation of spherical bubbles of 

 steam within them, and the upward movement and escape of these 

 steam bubbles, under the influence of hydrostatic pressure ; while in 

 the very coarsest grained or extremely crystalline lavas, from want 

 of sufficient mobility in their component particles, the steam only 

 escapes by filtration through and between these particles, causing the 

 sudden contraction and congealing of the mass and the formation of 

 gaping fissures; this process rapidly reaching to a considerable depth, 

 and the result being the separation of the surface into those more or 

 less loose and cuboidal blocks, often of large size, which I have 

 described above as characteristic of some lava-fields. 



I do not, however, find that this explanation of the difiierences in 

 question has met with much consideration or acceptance among 

 geologists. Tbe old notion still seems to prevail as to the equally 

 complete fusion of all lavas at the time of their emission, and that 

 their differences of texture are owing only to raore or less rapid 

 cooling, notwithstanding the certainty that the surface, at least, of 

 every lava-stream exposed to the oioen air must be subject to pre- 



