METAMORPHOSED SANDSTONE. 347 



observed in ]>()larize(l light that between the coarser waterworu fragments of colored, 

 eryi)tocry.stalline ([uartzite lies a mass of colorless ([iiartz in angular, closely fitting 

 grains, the salient angles of one corresponding to reentrant angles of those smronnd- 

 ing it. Upon dose examination in ordinary light each angnlar crystal is seen to 

 inclose a large round grain of (piartz, frequently full of tluid inclusioks and contain- 

 ing microlites and triehit«s, the narrow border being perfectly pui"e quartz. This 

 is illustrated in Fig. 3, PI. iv. From this it is evident that the rock is an ordinary 

 quartz conglomerate of rounded pebbles cemented together by silica that has crystal- 

 lized around the fragments of quartz crystals, taking tlie same crystallographic orien- 

 tation as the nucleus and thus extending the individual until obstructed by the 

 surrounding bodies. 



The same observations were tirst made and published l)y Tiirnebohni' in IS70 

 and subsequently were observed by H. Clifton Sorby- and i)ublished by him in an 

 address before the Geological Society of London, February 20, 1880. The same i)he- 

 nomenon was described by A. A. Young in the American Journal of Science for July, 

 1881; and still later, in 1883, R. D. Irving published in the same journal for June 

 a paper on the similar enlargement of quartz grains in the St. I'eters and Potsdam 

 sandstones and in certain Archean quartzites in Wisconsin, and in 1884 Irving and 

 Van Hise published a bulletin "on secondary enlargements of mineral fragments in 

 certain rocks," ^ in which, in addition to quartz, the enlargement of feldspars by the 

 same process of accretionary crystallization is described. 



The same thing has been observed by T. G. Bonney and Mr. J. A. Phillii)s in 

 England.^ 



' A. E Tornebohm, " Ein Beitrag zur Frage der Qnarzitbildung." Geol; Foren Stockh, 1876, vol. 

 m, p. 35. Reviewed in Neties Jahrbuoh fiir Min., etc., 1877, p. 210. 

 'Quart. Jnurn. Geol. Soc, Loudon, 1880, vol. xxxvi, p. 62. 

 " Bull. 8 of the U. S. Geol. Survey, 1884. 

 ■•Quart. Journ. Geol. Soc, London, vol. xxxix, p. 19. 



