Poulett-Scrope — On Mallet's Theory of Volcanic Energy. 31 



severed masses, or masses of heated dust, or so-called ash, but with- 

 out ejecta due to elastic steam The so-called deposits of 



"volcanic ash," the trap-dykes and porphyry -bosses of the Silurian 

 rooks in the south of Ireland and North Wales, etc., are evidences 

 of igneous action indeed, but of that hydrostatic character which 

 preceded the explosive volcanic action of the present epoch. 



Now there is certainly no reason to suppose the "ash," which is 

 so largely developed among the eruptive rocks of Silurian age, to owe 

 its origin or the comminution of its particles to any other cause than 

 that Avhich threw up and triturated the volcanic ash of more recent 

 formations. Mr. Mallet seems to consider the former to have been 

 produced in some obscurely hinted way under-ground (perhaps by 

 his crushing process), and to have " welled up " some how or other, 

 together with the "volumes of liquid rock (lava) " by "hydrostatic" 

 pressure, "without any ejections due to elastic steam." The notion 

 of such a process is not merely without reasonable foundation, but 

 is indeed unintelligible. That some of the plutonic crystalline rocks 

 " welled up " in a more or less liquefied state, through fractures in 

 the crust, unaccompanied by explosive ejections of scoriae or ash, 

 is highly probable, and seems proved by the absence of beds of such, 

 fragmentary character in their neighbourhood. Such eruptions 

 perhaps took place mostly under the ocean, the weight of water 

 above the vent preventing the outward explosion of steam bubbles. 

 And it is probable that similar formations are even now going on 

 among submarine volcanoes. But that the explosive and "hydro- 

 static " action of volcanoes belongs severally to different eras of the 

 earth's history, the one " earlier than the end of the Secondary age," 

 the other beginning then and extending to our own time, is wholly 

 imaginary, and contradicted by indisputable geological facts. 



This unauthorized notion of the existence of vast masses of "dust" 

 beneath the earth's crust pervades much of Mr. Mallet's theoretical 

 view of the cause of "hypogeal disturbances." He speaks of the 

 water of the ocean above penetrating through crevices to such masses 

 "of crushed and crumbled rock at a red heat," filtering through 

 them as water does through red-hot sand in our furnaces (§ 211), 

 and eventually boiling up as "boursoufSe lava." 



All this is of course mere conjecture. 



These examples are given here to show how imperfect are the data 

 on which Mr. Mallet bases his calculations as to the amount of heat 

 lost by the globe through the action of its volcanoes. But the data 

 for forming any opinion on the opposite side of the proposed 

 equation — viz. the amount of heat produced by the crushing of the 

 rocks composing the solid crust of the earth through their subsidence 

 from gravitation towards a shrinking nucleus — are of a character still 

 more vague and uncertain; so much so, indeed, as to render any 

 calculation upon them perfectly illusory. What reliable measure 

 can be obtained of the rate of cooling of the heated nucleus of the 

 globe ? of its consequent shrinkage ? of the crushing force thus 

 exerted on the more or less solid crust, or the amount of heat thereby 

 developed ? Calculation is wholly out of place where the data on 



