XIII.] ORIGIN OF CRYSTALLINE ROCKS. 317 



studies on metamorphism, which appeared in the Memoirs of the 

 Academy of Sciences of France (Vol. XVII.) ; and in it we find that, 

 consistently with the new views adopted by him in 1859, the old 

 doctrine of the epigenic origin of serpentine and the related mag- 

 nesian rocks from the alteration of plutonic rocks is abandoned. 

 In its stead, it is here suggested by Delesse that all these magnesian 

 rocks result from the crystallization of the sepiolites or so-called 

 magnesian clays, which are frequent in many sedimentary deposits. 

 These, according to him, by a molecular rearrangement of their 

 elements, may give rise to serpentine, talc, chlorite, and their 

 various associated and related minerals. The rocks thus generated 

 are still declared to pass insensibly into plutonic rocks ; but, instead 

 of maintaining, as in 1858, that they are derived from the latter, 

 Delesse, in 1861, asserts, on the contrary, that "the plutonic rocks 

 are formed from the metamorphic rocks, and represent the maximum 

 of intensity, or extreme limit of metamorphism." 



This recognition of the notion that the great masses of serpen- 

 tine, with their constantly associated hornblendic, talcose and 

 chloritic rocks, have been directly formed from the molecular re- 

 arrangement or diagenesis of aqueous magnesian sediments, and not 

 from the chemical alteration or epigenesis of erupted plutonic 

 masses, marks a complete revolution in our views of the history of 

 the crystalline rocks. The new doctrine did not, however, originate 

 with Delesse, but was previously put forward by myself in a paper, 

 On some Points of Chemical Geology, read before the Geological 

 Society of London in January, 1859, appearing in abstract in the 

 Philosophical Magazine for February, and published at length in 

 the Geological Journal for November, in the same year. I there 

 maintained that serpentines were " undoubtedly indigenous rocks, 

 resulting from the alteration of silico-magnesian sediments " ; and 

 moreover asserted that the final result of heat, aided by water, on 

 such rocks, would be their softening, and, in certain cases, their ex- 

 travasation as plutonic rocks ; which were regarded " as, in all 

 cases, altered and displaced sediments." When this paper was 

 written, in 1858, I still supposed that the reactions between the 

 elements in beds of silicious magnesian carbonates (which, I had 

 shown, may give rise to certain magnesian silicates in immediate 

 proximity to eruptive rocks) might serve to explain the origin of 

 great areas of serpentine and related crystalline magnesian sili- 

 cates ; but my studies of the silicates deposited during the evapora- 

 tion of natural waters, and of the magnesian sediments of the Paris 



