52 Pratt — Origin of the Corundum associated with 



Historical. — As was stated above, but little has been pub- 

 lished regarding the origin of corundum, although many 

 authors in their papers on the occurrence of corundum and the 

 peridotites mention what they consider might be the prob- 

 able origin of this mineral. 



In 1872, C. U. Shepard* in an extended article on the corun- 

 dum of North Carolina and Georgia, describes the occurrences 

 of corundum and chrysolite, the associated minerals and what 

 he calls the development of the "Strata" which "exhibit the 

 following order of formations: 1, Chrysolitic rock somewhat 

 mixed with anthophyllite ; 2. a layer of micaceous rock; 3. a 

 seam of chalcedony; 4. a stratum of chloritic rock (ripido- 

 lite) ; 5. the same through which the corundum is regularly 

 diffused, sometimes in narrow veins or widening out to several 

 feet." Nowhere in this article does he suggest the probable 

 origin of the corundum. 



J. Lawrence Smithf in describing the occurrence of corun- 

 dum associated with the peridotites of North Carolina and 

 Georgia, speaks of all the localities of corundum that he has 

 observed or examined as exhibiting certain prominent charac- 

 teristics common to all, but with each locality having its own 

 peculiar characteristics. He says that in all cases, however, 

 the masses of corundum give evidence of having been formed 

 by a process of segregation, which he has described in a 

 memoir;); on the Asia Minor emery ; that by the exercise of 

 homogenous and chemical attractions, the minerals which con- 

 stitute and are associated with emery were separated out from 

 the calcareous rock before it consolidated. 



Genth,§ although not touching directly on the origin of 

 corundum or giving any evidence to sustain his statement, 

 says, " that at the great period when the chromiferous-chryso- 

 lite beds were deposited, a large quantity of alumina was 

 separated which formed beds of corundum." This corundum 

 has subsequently been acted upon and altered and changed 

 into various minerals; that the "veins" of chlorite, etc. are 

 alterations of the original mass of corundum and that the 

 corundum might be found in a less altered or wholly unaltered 

 condition when the vein was explored below the action of sur- 

 face influences. In speaking of the corundum crystals imbed- 

 ded in the chlorite, Dr. Genth says the crystals " appear to 

 have formed after a great portion of the original corundum 

 has changed into chlorite, as if there had been an excess of 

 alumina ready for combination, which, not finding a supply of 



*This Journal, III, iv, pp. 109-114, 175-177, 1872. 

 f This Journal, III, vi, pp. 180-6, 1873. 

 \ This Journal, II, x, p. 354, 1850. 



§Proc. Am. Phil. Soc, xiii, pp. 361-406, 1873, and this Journal, III, vi, pp. 

 461-462, 1873 (review). 



