278 Revieics—Prof. T. Sternj Emit— 



minerals are far from being unknown. Yet the difficulties presented 

 by the removal of such large quantities of alumina are such as to 

 leave it very doubtful w^hether the substitution theory can be accepted 

 in the case of large masses of the nearly pure magnesian, or more 

 correctly speaking, ferromagnesian, hydrous silicate. 



11. The plutonic theory has of course many supporters, and may 

 on the whole be described as the one generally adopted. This again 

 is much subdivided.: some, like Daubree, maintaining the direct 

 ejection of the hydrous silicate ; others, like Bonney, that serpentine 

 results from the hydration of eruptive olivine rock : and there is also 

 another hypothesis which Hunt characterizes as hydroplutonic, and 

 which has many supporters among Italian geologists. This might 

 almost be placed in a separate category ; but as an eruptive origin is 

 claimed, it must be classed with the other plutonic theories. It 

 supposes that the material was ejected from the earth's interior, not 

 in a state of igneous fluidity, but as an aqueous magma or mud, 

 consisting essentially of a hydrous silicate of magnesia, which sub- 

 sequently consolidated into serpentine, and ultimately into olivine 

 and enstatite, thereby reversing the usual supposition as to the 

 hydration of these minerals having been the origin of serpentine 

 rock, which is itself regarded simply as a preliminary stage towards 

 the formation of the anhydrous silicates. 



It is not at all improbable that such eruptions of magnesian mud 

 may have occurred ; indeed, the very curious phenomena of the 

 South African diamond fields seems to be best explained on some 

 such hypothesis : nevertheless it merely postpones the difficulty one 

 stage, since such magmas could only proceed from a mass analogous 

 to, if not identical with serpentine, which was seated at some depth 

 below, and subjected to heat and pressure sufficient to cause an 

 extravasation with the constitutional water entangled in the viscous 

 mass and acting partly as the caiTier. Dr. Hunt regards these ideas 

 of the Italian geologists as merely an attempt to accommodate the 

 presumed igneous origin of serpentine with certain stratigraphical 

 conditions in which they are seen to occur. But his merriment at 

 the notion of a subterranean providence "which could send forth at 

 pleasure from its reservoirs alike granite and basalt, olivine rock 

 and limestone, quartz rock and magnetite," is somewhat beside the 

 mark, if indeed it is not a kind of device for producing a false im- 

 pression, by associating rocks for the most part of eruptive origin 

 with those that are not so. 



III. — The author himself asserts the aqueous origin of the masses 

 of native magnesian silicates, and their formation by reactions be- 

 tween the soluble silicates of lime and alkalies, derived from decay- 

 ing rocks, and the magnesian salts of natural waters. Thus would 

 be formed, we suppose, even the anhydi'ous magnesian silicates, just 

 as augite and the felspars may be formed in the wet way, Dieulafait's 

 notion is an improvement on Hunt's idea, since he regards serpen- 

 tines as having been formed in estuaries of the sea by the reactions 

 between the siliceous matters derived from the decay of pre-existing 

 rocks and the magnesian salts of sea-water. 



