Permanency op Clastic Characters in Rocks. 



The late Professor Irving, in the later years of his life, and I, as his assistant, 

 gave a good deal of time to investigating the permanency of the evidence of 

 clastic origin in rocks. It has been found that vitreous quartzites, for in- 

 stance, which formerly were regarded as metamorphic in the old sense, show 

 their fragmental character in the main as well as the day they were deposited. 

 About one hundred localities, the most of them of pre-Cambrian age, are 

 mentioned in Bulletin No. 8 of the U. S. Geological Survey, in which the 

 induration of quartzites was produced by a process of enlargement of old 

 quartz particles or else the deposition of new quartz between the grains 

 rather than a destruction of the original fragments. This list could at the 

 present time be greatly extended, and would include the larger quantity of 

 the Potsdam and post-Potsdam quartzites west of the Appalachian and east 

 of the Sierras, as well as most of those which have been designated as be- 

 longing to the Huronian. So far as our experience has extended, practically 

 all quartzites properly so called, of whatever age, thus reveal their fragmental 

 character, except when they have been subjected to great dynamic action. 

 It has been also found that feldspar, both monoclinic and triclinic, and horn- 

 blende* have the same power of renewed growth in fragmental rocks ex- 

 hibited by quartz grains. These phenomena have been observed both in 

 Keweeuawan and Huronian rocks. While locally important, enlargements 

 of this sort do not approach in their wide extension to that of quartz grains. 



It has been found that pressure alone, or, in other words, the weight of 

 any ordinary amount of superincumbent rock, has been wholly unable to 

 obliterate in the slighest degree the evidence of fragmental characters in 

 quartzites. For instance, a vitreous quartzite is found at the base of the 

 Peuokee series of Wisconsin. Above it is the whole thickness of the Peno- 

 kee series, some 12,000 feet, and over this the great Keweenawan series, 

 estimated by Irving to be 50,000 feet thick at the Montreal river, f It is 

 possible, and indeed probable, that the great synclinal movement which 

 formed the Lake Superior basin and exposed this vast thickness of rocks 

 began before the end of Keweenawan time. This being the case, these 

 quartzites cannot be asserted to have received the entire pressure of what 

 now appears to be the superincumbent mass of rock, but they must have 

 been buried many thousands of feet below the surface. However, the grains 

 of quartz now betray no evidence whatever of this. The particles are not 

 even arranged with their longer axes in a common direction. A quartzite 



* Enlargements of Feldspar Fragments in Certain Keweenawan Sandstones; C. R. Van Hise : U. 

 S. Geol. Survey, Bulletin No. 8, 1884, Part II, pp. 41-47. Enlargements of Hornblende Fragments; 

 C. R. Van Hise: Am. Jour. Sri., 3d Ser., Vol. XXX, 1885, pp. 231-235. 



fThe Copper-Bearing Rocks of Lake Superior, R. D. Irving, Monograph V, U. S. Geol. Survey, 

 1883, p. 230. 



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