254 ANNUAL OF SCIENTIFIC DISCOVERY. 



there existed in former periods a much greater intensity of causation 

 than that which now prevails. His theory is, that whereas now, in 

 the formation of beds, the aqueous action predominates, and the igne- 

 ous is only represented by a few solfataras, in the most ancient times 

 the action was much more igneous, and that in the intermediate 

 times fire and water divided the empire between them. In a word, 

 he concludes with the expression of the opinion, which my long-con- 

 tinued observation of facts had led me to adopt, " that the nature, 

 force, and progress of the past condition of the earth cannot be meas- 

 ured by its existing condition." 



In addition to these observations on metamorphism, let me remind 

 you that, on the recommendation of the British Association, other 

 important researches have been carried on by Mr. William Hopkins, 

 and in the furnaces of Mr. Fairbairn, on the conductive powers* for 

 heat in various mineral substances. Although these experiments 

 have been retarded by a serious accident which befel Mr. Hopkins, 

 they are still in progress, and I learn from him that, without entering 

 into any general discussion as to the probable thickness of the crust 

 of our planet, we may even now affirm, on experimental evidence, 

 that, assuming the observed terrestrial temperature to be due to cen- 

 tral heat, the thickness of this crust must be two or three times as 

 great as that which has been usually considered to be indicated by 

 the observed increase of temperature at accessible depths beneath the 

 earth's surface. 



Of the Devonian rocks, or old red sandstone, much might be said 

 if I were to advert to the details which have been recently worked 

 out in Scotland and in England ; but confining myself to general ob- 

 servations, it may be stated, that a triple subdivision of that group, 

 which I have shown to hold good over the continent of Europe, as in 

 our own country, seems now to be generally admitted. 



Very considerable advances have been made in the development 

 of the Carboniferous system. The close researches of Mr. Binncy, 

 who has from time to time thrown new lights on the origin and rela- 

 tions of coal, and the component parts of its matrix, established 

 proofs, so long ago as 1840, that great part of our coal-fields was ac- 

 cumulated under marine conditions; the fossils associated with the 

 coal-beds being, not, as had been too generally supposed, of fluviatile 

 or lacustrine character, but the spoils of marine life. Professor Henry 

 D. Rogers came to the same conclusion with regard to the Appa- 

 lachian coal-fields in America, in 1842. Mr. Binney believes that 

 the plant Sigillaria grew in salt water, and it is to be remarked that 

 even in the so-called "fresh-water limestones " of Ardwick and Le 

 Botwood the Spirorbis and other marine shells are frequent, whilst 

 many of the shells termed Cypris may prove to be species of Cythe- 

 rea. Again, in the illustrations of the fossils which occur in the 

 bands of iron ore in the South Welsh coal-field, Mr. Salter, entering 

 particularly into this question, has shown that in the so-called " Unio- 

 beds" there constantly occurs a shell related to the Mya of our 

 coasts, which he terms Anthracomya ; whilst, as he stated in the 

 Memoirs of the Geological Survey, just issued, the very Unios of these 

 beds have a peculiar aspect, differing much from that of true fresh- 

 water forms. They have, he said, a strongly-wrinkled epidermis, 



