XIII.] ORIGIN OF CRYSTALLINE ROCKS. 305 



similar conclusions. The views of the latter, as cited by 

 Credner from the work just referred to, are in substance as 

 follows : the crystalline schists, with their interstratified lay- 

 ers, have all the characters of altered sedimentary deposits, 

 and from their mode of occurrence cannot be of igneous ori- 

 gin, nor the result of epigenic action. The originally formed 

 sediments are conceived to have been amorphous, and under 

 moderate heat and pressure to have arranged themselves, and 

 crystallized, generating various mineral species in their midst 

 by a change, which, to distinguish it from metamorphism by 

 an epigenic process, Gumbel happily designates diagenesis* 



It is unnecessary to remark that these views, the conclusions 

 from the recent studies of Gumbel in Germany and Credner in 

 North America, are identical with those put forth by me .in 

 1860.t 



[* The following is extracted from an essay by the author in the Report 

 of the Smithsonian Institution for 1869, on The Chemistry of the Earth, 

 33 : " The gradual transformation of amorphous precipitates under water 

 into crystalline aggregates, so often observed in the laboratory, appears to 

 depend upon partial solution and re-deposition of the material, which must 

 not be entirely insoluble in the surrounding liquid. If the solvent power of 

 this be reduced, the dissolved portions are deposited on certain particles 

 rather than others. By a subsequent exaltation of the solvent power of the 

 liquid, solution of a further portion takes place, and this, in its turn, is de- 

 posited around the nuclei already formed, which are thus augmented at the 

 expense of the smaller particles, until these at length disappear, being gath- 

 ered to the crystalline centres. Such a process, which has been studied by 

 H. Deville, suffices, under the influence of the changing temperature of the 

 seasons, to convert^ many fine precipitates into crystalline aggregates, by the 

 aid of liquids of slight solvent powers. A similar agency may be supposed 

 to have effected the crystallization of buried sediments, and changes in the 

 solvent power of the permeating water might be due either to variations of 

 temperature or of pressure. Simultaneously with this process one of chemical 

 union of heterogeneous elements may go on, and in this way, for example, we 

 may suppose the carbonates of lime and magnesia become united to form 

 dolomite or magnesian limestone."] 



[t Since the first publication of the above address I have received in a pri- 

 vate letter from Gumbel the following re-statement of his views as to the 

 origin of crystalline rocks : " I have seen no occasion to change my opinions, 

 which are, I believe, identical with your own. I do not maintain a metamor- 

 phic origin for the primitive rocks ; for, although these are certainly much 

 altered, there are no firm and consolidated rocks which are not so. They 

 were formed like, for example, the limestones of more recent periods ; these 



T 



