530 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 2, 



or by other later observers, inconsistent with the supposition that 

 they are solid throughout, and have all originated in the usual manner 

 by the heaping up of erupted lavas and fragmentary ejecta above their 

 respective vents. Many of them have been seen in eruption, ejecting 

 vast quantities of fragmentary pumice (felspathic scoriae) and ashes, 

 as well as currents of pumice or obsidian, similar to those of some 

 of the most evident eruptive cones and craters, for example, of 

 Lipari, Volcano, and YolcaneUo. Others seem likewise to have 

 given birth to radiating streams of trachytic lava, which, on cooling, 

 have outwardly split into massive incoherent blocks. These streams 

 are called trainees de blocs by Humboldt and Boussingault, and are 

 considered by them to have been erupted in this disaggregated con- 

 dition from fissures* ; but it would be more in accordance with the 

 analogy of ordinary volcanic phenomena to look upon them as true 

 lava-currents. Many of the felspathic streams of the Monts Dome 

 and Yelay, and indeed of Etna, Vesuvius, and Heclaf likewise, and 

 some of the basalts of the Siebengebirge, consist superficially of a 

 chaos of apparent fragments — heaps of loose cuboidal blocks, evi- 

 dently resulting from a divisionary shrinkage and fissuring of the 

 superficial lava on exposure to the air, from the rapid escape of the 

 disseminated vapour to which in great part it owed its imperfect 

 fluidity. 



In extreme cases this splitting into loose blocks on consolidation 

 has been carried to so great a depth from the surface that, in the 

 absence of actual cliif-sections, they seem to constitute the entire 

 mass, especially towards the sides and termination of the current ; 

 and as they were, no doubt, in motion, tumbhng over one another, 

 as the lava flowed on, they will necessarily bear the appearance in 

 such situations of a stream of fragments (trainee de blocs). M. de 

 Humboldt's idea of their having been erupted in this fragmentary 

 form is quite imaginary; and M. Eoussingault's notion of earth- 

 quakes being caused by the rattling of such loose blocks in the 

 interior of mountains J (like dice in a caster) is still more far-fetched 

 and untenable. Professor Dana describes the greater number of lava- 

 fields in Hawaii as composed of such loose angular blocks of all 

 shapes, and of sizes from that of a half-bushel measure to that of a 

 house, possessing a surface of horrible roughness. He calls them 

 *' Clinker-fields," and attributes their peculiar character to the lava- 

 stream having been broken up during consolidation of the surface by 

 a fresh moving impulse §. Such too, for the most part, are the lavas 

 which fill the Caldera of Teneriffe, as may be seen in the photographic 

 views of Professor Piazzi Smyth's recent volume. 



Others of the trachytic domes of the Cordilleras (the '^ unopened 

 domes or bells" of M. de Humboldt) probably swelled up in pasty or 

 viscous masses above their vents of eruption, hke the Puy de Dome 

 and its associated bosses. The idea of their being hollow, and having 

 risen like a bhster, is not supported by anything known of their 

 structure, nor, as I have already shown, by the example of any 



* Kosmos, iv. pp. 310-318. t See Bunsen's ' Iceland *. 



t Kosmos, iv., note to p. 170. § U. S. Expl. Exp. i. p. 162. 



