METAMORPHOSED SANDSTONE. 347 



observed in polarized light that between the coarser waterworn fragments of colored, 

 cryptocry stall iue quartzite lies a mass of colorless quartz in angular, closely fitting 

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

 ing it. Upon close examination in ordinary light each angular crystal is seen to 

 inclose a large round grain of quartz, frequently full of fluid inclusions and contain- 

 ing inicrolites and trichites, the narrow border being perfectly pure 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 the same crystallographic orien- 

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

 surrounding bodies. 



The same observations were first made and published by Tornebohni 1 in 1876 

 and subsequently were observed by H. Clifton Sorby 2 and published by him in an 

 address before the Geological Society of London, February 20, 1880. The same phe- 

 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. Peters 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," 3 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. Phillips in 

 England. 4 



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

 Ill, p. 35. Reviewed in Neues Jahrbuch fur Min., etc., 1877, p. 210. 

 'Quart. Journ. Geol. Soc., London, 1880, vol. xxxvi, p. 62. 

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

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



