MINERALOGY AND PETROLOGY. 971 



Bareness of shearing and crushing phenomena.] 



where usually no trace of an earlier state of the constituent minerals remains, those 

 principles would hardly serve in the treatment of a muscovadyte in which the same 

 minerals can be traced to prior states, nor to the still earlier condition where meta- 

 morphism has been so slight that the new minerals can hardly be found, but where 

 a wholly uncrystalline condition pervades the basic debris. 



These three difficulties have sometimes been obviated by the non-use of the 

 terms, or by a special description or qualification. If a term could be agreed upon 

 to designate the permanent minerals of an igneous rock which originated during the 

 cooling state of the rock, its use would promote exact definition and be of service to 

 petrographers. 



Rareness of shearing and crushing phenomena. The writer has found little reason 

 to ascribe the present condition of the Archean minerals to shearing and crushing 

 subsequent to their formation. These dynamic features are comparatively rare. 

 The shearing and pressure which acted in Archean time seem to have completed 

 their task, and very largely promoted, if they did not provoke, the recrystallization 

 which characterizes the Archean minerals. Such recrystallization completed, the 

 minerals have subsequently, almost without exception, preserved their crystalline 

 integrity to the present day. In their perfection the minerals of the Lower Keewatin 

 went into the conglomerate of the base of the Upper Keewatin, and, aside from the 

 decay incident to the long-continued exposure which the formation of that conglom- 

 erate implies, they have persisted till the present, and are identifiable as having 

 come from the Lower Keewatin. Again, the whole Archean, including Upper and 

 Lower Keewatin, was buried under the Taconic, and where the Taconic has been 

 removed or penetrated by drills, the Archean minerals are as perfect, apparently, as 

 when they were so buried. The gabbro and the diabases of the Keweenawan, as well 

 as the rhyolytes and obsidians, when not locally subjected to abnormal conditions, 

 are apparently as perfect in their crystalline and chemical conditions as they were 

 at the moment they acquired their normal temperature.* Throughout all the body 

 of the crystalline rocks, whether of the Archean or of the Taconic, there is little sign 

 either of shearing, crushing, or of decay. The slumber of unchangeableness seems 

 to have settled upon them, awaiting the advent of some great convulsion to start 

 again their energies by imposing conditions of non-equilibrium.f 



While stability and long endurance are the characters most easily legible in 

 the oldest crystalline rocks, it is necessary to admit that the Archean rocks have 

 undergone many changes, but those changes are in great measure but local and 

 temporary effects of special conditions or exceptional forces. Belts of folding and 



*In rock No. 17tJ glass still subsists. 



t Such instances as rock No. 1011, which may b- assumed to be an old Archean diabase, and which contains no quartz, 

 though much altered, rather show that when quartz does enter a diabase it is due neither to ago, nor to surface e.\)x>sure, nor to 

 normal decay (even with dynamic action), and may hence be attributed to that source which is directly suggested in many cases, 

 viz., endomorphism from contact on acid elastics. 



