bearing on theoretical Speculations. 115 



complete accounts of volcanos extant, published in the Dic- 

 tionnaire des Sciences Nalurelles : 



" Pour rasseinbler ici toutes les observations qui concourent 

 a etablir LE REPOS ACTUEL des grands phenomenes geolo- 

 giques, et Pabsence de toute formation complete de terrain 

 nouveau, je dois rappeler en abrege les faits qui dans Phis- 

 toire des terrains pyrogenes volcaniques tendent a confirmer 

 cette proposition. 



" 1. Toutes les bouches volcaniques en activite fontpartie 

 d'une systeme volcanique dont Porigine ou Pepoque d'appa- 

 rition a la surface du globe est absolument inconnu. On ne 

 pent citer aucuiie nouvelle bouche volcanique, aucun nouveau 

 cone ou butte volcanique terrestre, littoral, ou marin, qui ne 

 fasse partie ou ne soit lie avec un systeme de terrain volcanique 

 ancien." p. 61. He adds, that the modern lavas are destitute 

 of many rocks and minerals which characterize the ancient, 

 e.g. true basalt, trachyte, metallic minerals, &c. ; and that the 

 rocks which are produced by aqueous solution, such as the 

 siliceous incrustations of the Geyser, and concretionary lime- 

 stones in modern volcanic districts, are little varied and of 

 small extent compared with the analogous deposits of the an- 

 cient basaltic and trachytic tracts. 



I have been the more anxious fairly to state these argu- 

 ments, because in a very valuable publication, which has ap- 

 peared at the moment I am writing, by one of our first scien- 

 tific names (which I had most earnestly hoped to have seen 

 placed, where it undoubtedly ought to have been, at the head 

 of our first scientific Society) , I mean Herschel's Discourse on 

 the Study of Natural Philosophy, p. 147, the author, speaking 

 of the diminution of the temperature of the globe, states that 

 some geologists have ascribed this to the immensely superior 

 activity of former volcanos, which, however, he observes, can 

 hardly be esteemed a vera causa; for, says he, " we are not 

 sure that such supposed greater activity of former than of pre- 

 sent volcanos really did exist." Now, unless I have entirely 

 failed, I would hope that in my late communications I have 

 shown some grounds for believing that we are as sure of this 

 fact as we can be of any which is not submitted to the evi- 

 dence of the senses, but requires to be established by a chain 

 of inferential reasoning. Mr. H. himself inclines to refer this 

 diminution of temperature to the diminution of the excentri- 

 city of the earth's orbit. On a subject of this nature I feel 

 myself altogether incompetent to hazard an opinion ; and I 

 will therefore only inquire whether, seeing that the mean 

 distance of the earth from the sun is necessarily a constant in- 

 variable amid all the oscillations of the system, the proposed 



Q 2 secular 



