Reviews — Dr. T. Sterry Hunt's Chemico- Geological Essays. 557 



In discussing " The probable seat of Volcanic Action," Dr. Hunt 

 alludes to a modification of the theory of the liquid zone between the 

 solid core and the crust, which he is inclined to adopt, whilst accepting 

 in the main the theory of Hopkins. This theory of Sterry Hunt is 

 Neptunean in the highest possible degree, but there is one point on 

 which he has always been staunch — viz. that the originating heat is 

 derived from the central mass. Indeed, he thinks it probable that 

 any chemical processes which may be set up in the buried sediments 

 for their conversion into igneous rocks and volcanic products would 

 absorb rather than generate heat. As some people, perhaps from an 

 instinctive dread of telluric heat, have exercised much ingenuity of 

 speculation on the causes of the heat of vulcanicity, it is satisfactory 

 to find that Hunt, though a Neptunist, has no difficulty on this point. 

 But he considers that, although in early times a yet un solidified sheet 

 of molten matter may have existed between the solid core and the 

 superficial crust, such is not now the case. He believes that the 

 cushion on which the flexible crust undulates is in fact the base of 

 that crust impregnated with water and in a state of aquo-igneous 

 fusion. When there is an excess of pressure, this may tend to pro- 

 duce fluidity — the very reverse of Scrope's idea that relief of 

 pressure effected this. Hence also the belief of Hunt that the 

 phenomena of volcanic eruptions are the most likely to occur under 

 the more recent formations, where he appears to conceive that there 

 is the greatest amount of pressure, and he endeavours to show, 

 though perhaps not very successfully, that the present distribution 

 of volcanic areas favours this view. Certainly the ancient volcanos 

 in Auvergne and the Eifel, piercing as they did rocks of high 

 antiquity, do not substantiate his case ; and this is the more important, 

 as Dr. Hunt seems to predicate for the old crystalline areas an 

 immunity from volcanic disturbance in future. 



The composition of the semifluid cushion is of course all impor- 

 tant. The existing crust being the water-deposited debris of the 

 primeval slag mixed with such precipitates as have from time to time 

 been abstracted from the liquids which condensed upon it, contains 

 all the substances and may yield all the phenomena of volcanic 

 eruptions. Hence it may be deemed self-containing, and requires no 

 reinforcement from below, the central solid core acting merely as a 

 stove to supply heat. There is no necessity therefore for the acidic 

 and basic magmas of Durocher and others, and the admitted division 

 of the rocks, both volcanic and crystalline, into what corresponds on 

 the whole to trachytic and doleritic, is capable of another explanation, 

 which also will better accord with the complex nature of the rocks 

 themselves. Such a separation has in fact been going on from the 

 very earliest times, owing to the effects of atmospheric waters, which 

 remove from the rocks soda, lime, and magnesia, leaving behind 

 silica, alumina and potash — the elements of granitic and trachytic 

 rocks. This action is more or less complete, according to the perme- 

 ability of the rocks, and thus arises a tendency to a division into two 

 classes of silico -argillaceous rocks constituting the bulk of the earth's 

 crust, — doubtless an ingenious suggestion, though hardly meeting 

 cases such as that of the abundance of alumina in anorthic felspars. 



