Correspondence — G. Poulett Scrope. 237 



represented as of Caradoc age in tlie Geological Survey Map, he found 

 a series of Trilobites and other fossils, which induced him to regard 

 these Shineton shales as belonging to the Lower Tremadoc series. 

 He described as new species : Asaphus Eos, Conocoryphe Salteri, C. 

 anguU/rons, Platypeltis Croftii, Conophrys Salopiensis, Lichapyqe cus- 

 pidata, Lingulella Nicholsoni, Metoptoma Sabrinoe, and Theca Uneata. 

 The author regarded these shales as the equivalents of beds contain- 

 ing Dictyonema found near Malvern and at Pedwardine. 



Discussion. — Mr. Etheridge differed entirely from the author, and thought the 

 fossils exhibited by him were of Caradoc age. 



Mr. Hicks was inclined to refer the fossils to the Upper Llandeilo, but the frag- 

 ments exhibited were not sufficient to enable the species to be determined. 



cois-E-iESiFOisriDEisroiE. 



THE SOURCE OF VOLCANIC HEAT. 

 ■ SiK, — Were it not for the sincere respect I entertain towards Mr. 

 E. Mallet, I should say there is something ludicrous in the persistence 

 with which he continues to saddle me with the advocacy of one or 

 both of what he, writing ex cathedra, pronounces to be " the mori- 

 bund theories of a thin crust and liquid nucleus, or of Hopkins's 

 fiery lakes." You have already, in the note to his paper, referred 

 to proofs that I have rather thrown doubts on both these hypotheses 

 than accepted them. I have been always slow to dogmatize upon 

 the condition of what is so glibly called the nucleus of our globe, 

 seeing how little we can know of it beyond the mere skin, and how 

 ignorant we are of the laws through which extremes of temperature 

 and pressure (not to mention other agencies, such as electric and 

 magnetic currents, diffusion of fluids, imperfect elasticity of solids, 

 solution of solids in fluids, and other chemical changes) ' may 

 affect the condition of the interior, even on the hypothesis of 

 its condensation from nebular matter. 1 have rather, from the 

 first, leant to the opinion that the immediate sub-cortical matter 

 of our globe, nnder varying conditions of heat and pressure, not 

 unfrequently passes, locally, from a solid to a fluid, and even a 

 partially vaporous state, and back again. All that I have contended 

 for, and that rather as an undeniable fact than a theory, is that there 

 must exist, within and beneath the vents of active volcanos, consider- 

 able masses of more or less liquid matter at an intense temperature 

 (lava), and of indeterminate extent, whether in a vertical or lateral 

 direction. And this much, if not more, Mr. Mallet himself must 

 believe; otherwise, where is he to get the fused matter which he con- 

 siders to have been forced upwards by hydrostatic pressure to form 

 the immense m.asses of plutonic rocks ? 



The only question really at issue between Mr. Mallet and myself 

 relates to the source of the intense internal heat so evidenced in sub- 

 terranean lava masses ; that is, whether it is derived chiefly and 

 directly by conduction or convection, or both, from the heated 

 nucleus of the globe (which Mr. Mallet himself postulates), or 



^ See a paper by Sir W. Thomson on the Dissipation of Energy, in Nature of 

 April 9th, 1874. ' 



