704 BEroKT— 1887. 



distinct Upper Taconic being an uncrystalline series of fossiliferous Cambrian 

 strata. 



The writer in this connection recalled the work of Neri, Gerlacb, and others in 

 the western Alps, and that of Von Hauer and his associates in the Lombardo- 

 Venetian Alps, where the same distinction of the true pietre verdi zone between 

 the ancient p^neiss below and the recent gneiss above had, unknown to him, been 

 pointed out by the Austrian geological survey two years before the present writer 

 in 1870 defined and named the younger gneissic series in North America. The 

 absence of the true pietre verdi series in some localities, alike in the Alps and in 

 North America between the older and younger gneisses was noted, as well as the 

 existence of apparent discordances between each one of the four great divisions of 

 Archaean or pre-Cambrian crystalline rocks above named. 



3. Mcments of Primary Geology} By T. Sterrt Hunt, LL.D., F.E.S. 



The author, after recalling his classification of original or non-clastic rocks into 

 Indigenous, Endogenous, and Exotic masses, based on their geognostic relations, 

 gives in a concise form his theory of the genesis of these various groups of rocks, 

 as taught more at length in his recent volume entitled ' Mineral Physiology and 

 Physiography.' The superficial portion of a cooling globe, consolidating from the 

 centre from a condition of igneous fusion, he conceives to have been the protoplas- 

 mic mineral matter, which, as transformed by the agencies of air, water, and in- 

 ternal heat, presents a historj' of mineralogical evolution as regular, as constant, 

 and as definite in its results as that seen in the organic kingdoms. This great trans- 

 formation involves a series of processes, which include, (1) the removal from the 

 protoplasmic mass, through permeating waters, heated from beneath, of the chief 

 elements of the great groups of indigenous crystalline and colloidal rocks, by what 

 he has called the crenitic process ; (2), the modification of the residual portion by 

 this lixiviation, which removes silica, alumina, and potash, and, by the intervention 

 of saline waters, brings in fidditioual portions of lime, magnesia, and soda ; (3) the 

 partial diilerentiation by crystallisation and eliquation, of portions of this more or 

 less modified residue, giving rise to the various types of plutonic rocks. The direct 

 and indirect results of subaerial decay through atmospheric agencies, and those 

 of the products of organic hfe, are also considered. From the operation of all these 

 processes result progressive changes in the composition alike of plutonic and of 

 indigenous rocks. The endogenous rocks or veinstones are, like the last, of crenitic 

 origin, and may be granitic, quartzose, or calcareous in their characters. 



The author next considers the conditions of softening and displacement of indi- 

 genous rocks which permit them to assume in many cases the relations of exotic 

 rocks, and to become extruded after the manner of lavas, as seen in the case of 

 trachytes and many granite-like rocks. Such masses he designates pseudoplutonic. 

 With these are often confounded the endogenous granitic veinstones, which were 

 formed under similar chemical conditions to the indigenous granites. Masses ahke 

 of indigenous, endogenous, and exotic rocks may also become displaced, not through 

 softening, but by being forced while in a rigid state through movements in the 

 earth's crust, among masses softer and more yielding than they. 



The author also considers the fluxional structure seen in iilutonic and pseudo- 

 plutonic eruptive masses, which has led some theorists to regard these as of aqueous 

 origin, and others to maintain that the crenitic stratiform masses themselves are of 

 plutonic origin ; two opposite errors which vitiate much of our geological literature. 



The crenitic process, by removing from beneath what was tlie original surface of 

 deposition, the vast amount of material which forms alike the indigenous, the 

 endogenous, and tbe pseudoplutonic rocks, has eflected a great diminution in 

 Yolume in the protoplasmic mass, besides that due in later times to extrusion of 

 plutonic matter itself. This decrease in bulk of the underlying stratum was a potent 

 agent in producing the universal corrugation of the earlier crenitic rocks, and the 

 frequent discordances observed among them. 



' rublished in extenso in the Geol. Mag. for November, 1887. 



