558 THE AZOIC system and its subdivisions. 



in regions where rocks of this kind occur. Not that all gneisses are of 

 this character ; but those are ordinarily so which with granite make 

 up the axial masses of disturbed regions. That the parallel structure 

 of the materials forming gneiss is not necessarily the result of sedimen- 

 tation seems to us clearly to result from that which has been done both 

 in experimental and field geology within the last few years. It can- 

 not be denied that a foliated arrangement or a parallel disposition of 

 the mineral elements of various sedimentary rocks can be, and often 

 has been, induced in them after their deposition, and that this parallel 

 arrangement is not by any means necessarily coincident with tlie planes 

 of stratification. This fact alone is absolutely conclusive in favor of 

 the idea that parallel arrangement of the mineral constituents of a rock 



— in other words, a gneissic structure, in rocks of the granitic flxmily — 

 is not proof of sedimentation. 



Overlying the granitic and gneissic axial rocks we are likely to find 



— and in many cases do find — the stratified masses which were formed 

 from the pre-existing crust themselves usually highly metamorphosed, 

 because formed at a period of great cliemical and mechanical activity. 

 With these stratified and highly altered masses are associated eruptive 

 materials — both interbedded and injected in dike-form — these also 

 often greatly metamorphosed, and to such an extent that their original 

 character is only with difficulty, and with the aid of the microscope, to 

 be recognized. This protrusion or forcing out of eruptive materials 

 seems to have followed the preceding uplift of the original crust, if not 

 as a necessity, at least as something extremely likely to occur, as is 

 shown by the fact that in so many great mountain chains we find vol- 

 canic activity more and more predominating with the progress of geo- 

 logical time. Since these eruptive materials come from a gradually in- 

 creasing depth below the surface of the original crust, they are more 

 basic than this, and, since as a rule they contain more iron than that 

 crust, are darker-colored than the masses by which they are directly 

 underlain. Hence the detrital beds formed from the debris of these 

 more basic materials are themselves of a dark color, and as a result of 

 their metamorpliism we have the various slates, argillaceous, talcose, 

 and chloritic, which so commonly rest upon the granitic and gneissoid 

 rocks which form the core or axis of the d'sturlied region. "With these 

 slaty rocks are also associated limestone masses, which — so far as our 

 observations go — are not ordinarily interstratified with the slates, but 

 are of the nature of segregated deposits, having been formed poste- 

 rior to the formation of the sedimentary beds with which they are asso- 



