468 Notices of Memoirs — Dr. T. Sterry Hunt — 



forth by Von Lasaulx and by Knop. This third hypothesis then' 

 proposes to account for the presence of various exceptional varieties 

 of rock among ordinary phitonic formations, by supposing that 

 limited portions of these have at different times been the subject of 

 very unlike chemical processes, resulting in their complete change 

 into new forms of rock by what has been called pseudomorphic alter- 

 ation, or metamorphosis. As, however, such a conversion involves 

 a change not only of form, but of substance, it has been more pro- 

 perly designated a metasomatosis. 



We have next to consider the Neptunean views as ordinarily ex- 

 pounded. This, while it accounts by sedimentation for the stratiform 

 arrangement of the crystalline rocks, and explains the existence 

 therein of beds of iron ores and limestones, still presents many of 

 the difficulties which are encountered in the Plutonist view. If, as 

 most Neptunists maintain, the great crystalline series have been 

 derived from the alteration of uncrystalline ones, which were not 

 only similar to those of Palseozoic and more recent times, but are, in 

 fact, portions of those which in adjacent regions are still known to 

 us in their original unchanged condition, how are we to explain the 

 genesis of the felspathic and hornblendic rocks which predominate in 

 these crystalline formations ? The sandstones and shales from which 

 in this view they are supposed to be formed could never by them- 

 selves give rise to the rocks in question, since they are deficient in 

 the alkalies, and to a greater or less extent in the other bases 

 required for the production of the constituent silicates. To explain 

 their origin, therefore, it becomes necessary to admit the intro- 

 duction of these various bases from without, and to suppose a series 

 of metasomatic processes more wonderful than those imagined by 

 the Plutonist. The latter, by his hypothesis, has already at hand 

 felspathic and hornblendic rocks, which are to be the subject of 

 metasomatosis, while the Neptunist has only the products of their 

 decay. In either hypothesis, we have to account for the presence in 

 the primary formations of beds and interstratified masses of a great 

 number of exceptional silicated rocks, very distinct in composition 

 from any mechanically-formed sediments, including not only silicates 

 like serpentine, olivine, steatite, chlorite, pinite, garnet, epidote, 

 and hornblende, but of pure orthoclase, as well as of triclinic 

 felspar. Each of these species would require for its production 

 from any ordinary igneous or aqueous rock a separate and inde- 

 pendent metasomatic process, involving the addition of certain 

 elements, and the abstraction of others, until the whole hetero- 

 geneous crystalline series was complete. The author illustrated 

 these views by examples from recent writers, and concluded that 

 the hypothesis of metasomatosis, as maintained both by Plutonists 

 and Neptunists, supposes the operation in solid rocks of processes 

 of circulation, absorption, elimination, selection, and aggregation 

 scarcely to be equalled in the economy of highly-organized beings, 

 and not easily imagined in the masses of the mineral kingdom. 

 Certain geologists suppose the existence of two classes of crystalline 

 stratified rocks, the one Neptunean, and consisting of altered portions 



