MINERALOGY AND PETROLOGY. 973 



Gradations in crystalline structure.] 

 Alteration of quartzyte. 



the Archean can be explained on the assumption that the intrusive rocks are 

 metamorphosed conditions of the clastic rocks near adjacent to the observed 

 intrusions, rendered plastic by the forces of dynamic metamorphism accompanied 

 by moisture. As an illustration of this transformation of the Archean elastics some 

 of the field phenomena of the region of Kekequabic lake were enumerated, and in 

 connection therewith is a statement of some of the petrographic facts that accom- 

 pany and confirm such transition.* The structural relations have also been described 

 in vol. iv, of this report (chapter descriptive of Lake county), and hence it will be 

 necessary here only to mention other petrological facts and inferences bearing on 

 that subject. 



1. Gradations in crystalline structure from granite to graywacke. It is evident 

 that, while it might be very infrequent to observe petrographic transitions from 

 crystalline to sub-crystalline and to fragmental, in situ, owing to the easily movable 

 nature ol the plastic or fluid rocks when under the pressure and shearing that pro- 

 duced the assumed plasticity, yet there ought to be found more readily all the 

 varying stages of recrystallization, when the rocks are studied in detail from different 

 localities. Such is the case. One of the most common instances in incipient recrys- 

 tallization is seen in the graywackes. If the reader will consult the descriptions of 

 the following rock numbers he will find some of the details of this progressive 

 crystallization, viz.: Nos. 341, 2184, 2244, 2245, 2264, 2269, 383G, 386G, 407G, 8H, 389H. 



General statements concerning the behavior of the different minerals con- 

 cerned have been made in the foregoing notes on the rock-forming minerals. It is 

 obvious that under such conditions of alteration and subsequent recrystallization it 

 is impossible sometimes to affirm unqualifiedly that the clastic rock was a typical 

 graywacke. It might have been somewhat pebbly, like a conglomerate, or too fine 

 to bear the name of graywacke. It might have been porphyrel or arkose; but the 

 special designation of the clastic rock is not important in this connection. What- 

 ever the correct appellation for the rock, the kind of change, tending toward a 

 granitic structure, is the same in all the clastic rocks when they encounter these 

 physical conditions. 



2. The same kind of alteration of quartzyte is described in connection with the 

 following rock numbers: Nos. 609, 784, 1724, 1839, 1840, 1852, 1853, 1854, 436H. In 

 this series No. 1852 is specially interesting, because of the favorable exposure and 

 the association of the recrystallized quartzyte (Animikie) with a black mica schist 

 which was produced from the Animikie slates under the same forces. The following 

 is from the writer's field book, July 5, 1893: 



" 1854. I have been on the lookout for evidence to show what becomes of the black slates when subjected 

 to the influences which make <|uartz-porphyry and red granite of the quartzyte, and I have only found that they 



*An earlier discussion of the same subject by the writer was presented to the American Association for the Advance- 

 ment of Science, August, 1898, and published in the American Geologist, November, 1898, vol. xxii, pp. 299-310. 



