454 0. B. Van Ilise — Origin of the Mica-Schists and Black 



show biotitic and chloritic greywackes. Continuing westward, 

 the rocks become more and more micaceous, and at Penokee 

 Gap, Wisconsin, they are mostly mica-schists and black mica- 

 slates, although narrow belts of micaceous greywacke are yet 

 found ; while the schists at English Lake taken b} r themselves 

 would probably be considered completely crystalline rocks — as 

 completely crystalline as some mica-schists in the older gneissic 

 formation. These rocks, at present of such widely varying 

 character, microscopic study shows to have been originally in 

 essentially the same condition. All were once, as some are 

 still, complete]}^ fragmental rocks composed chiefly of quartz 

 and feldspar, mingled in places with a little clayey matter, 

 perhaps also with a small quantity of fragmental mica and. 

 some ferrite. The fragmental feldspar is or was very largely 

 orthoclase, although plagioclase is usually, and in certain locali- 

 ties quite plentifully, found. Measurements of its extinction 

 angles show this plagioclase to be of an acidic character, and it 

 is probably in great part oligoclase. The proportions and mag- 

 nitudes of the fragments of quartz and feldspar vary in differ- 

 ent localities. The quantity of feldspar was apparently con- 

 siderably greater in the rocks of the western part of the area 

 than in those of the eastern part, i. e., was greater in those 

 rocks which are now mica-schists and mica-slates than in those 

 which have formed the kinds to which the name greywacke is 

 here applied. 



As far as the quartzites, greywackes and greyvracke-slates 

 are concerned the microscope shows that they reached their 

 present conditions by processes already fully described by Irving 

 and myself.* Briefly summarized, these processes included : 

 secondary enlargement of the quartz fragments and in a few 

 cases apparently of the feldspar fragments also; the deposition 

 or formation in situ of interstitial finely crystallized quartz ; and 

 a chloritic replacement of the feldspars, with consequent sep- 

 aration of silica. In general, the processes by which the mica- 

 schists and black slates have reached their present condition 

 are much the same as the above, with the very important dif- 

 ference that the feldspars instead of altering to chlorite have 

 altered to muscovite and biotite — chiefly the latter — with an 

 accompanjnng separation of silica ;f the result being the produc- 

 tion, from a completely fragmental rock, by metasomatic changes 

 only, of a rock which presents every appearance of complete original 

 crystallization, and which would be ordinarily classed as a genuine 

 crystalline schist. 



* Bulletin U. S. Geol. Survey, No. 8. 



f Lehmann in his work on the " Entstehung der altkrystallinischen Schiefer- 

 gesteine," demonstrates the formation of abundant secondary biotite and other 

 minerals as accompanying metamorphoses by folding. His work does not state, 

 however, exactly how the biotite developed. 



