ROGERS, GEOLOGY OF THE CORTLANDT SERIES 79 



Corundum is found in Xorth Carolina in peridotite segregated at the 

 contact of the igneous rock with gneiss. There is a sharp contact with 

 the gneiss, but the corundum grades into the peridotite. Pratt 68 be- 

 lieves that it separated out of the magma as an original constituent, ac- 

 cording to the laws cited above, and segregated at the border in a fashion 

 analogous to that of pyrrhotite or ilmenite. The earlier writers, 69 how- 

 ever, believe that it is due to contact action on the gneiss. Corundum 

 is also found in plumasite in Plumas County, California, and there is 

 no doubt that it is an original pyrogenic constituent. 70 



Corundum has been found on the contact of granite and clay slate on 

 Dartmoor in Devonshire, apparently due to contact action, 71 and at con- 

 tacts between granite and micaceous quartzite near Morlaix, France, 

 having the same origin and associated with sillimanite and andalusite, 

 spinel, etc. 72 



Corundum in sapphire crystals has been found in a monchiquite dike at 

 Yogo, Montana. The rock is too low in alumina, however, for it to have 

 separated out as an original constituent, and it is probably due to inclu- 

 sions of argillaceous sediment derived from the underlying rocks. 73 Co- 

 rundum and zircon found in basalt and other basic rocks in Haute Loire 74 

 are thought also to be due to the destruction of gneiss inclusions. 



Corundum, then, in igneous rocks, may be due to magmatic segregation, 

 or to contact action, or to the absorption of aluminous inclusions. 



Evidence as to Its Formation in the Cortlandt Series 



It has been shown above that the emery in the Cortlandt Series is often 

 streaky in appearance; usually associated with inclusions of mica schist 

 or found near the borders of the area but not on them ; immediately asso- 

 ciated with sillimanite rocks and other abnormal varieties; showing no 

 marked preference for any country rock as previously supposed, though 

 found most generally in pyroxenite, and often associated with rocks which 

 have undergone metamorphic action. Spinel and corundum may be scat- 

 tered through the adjoining rocks for a few yards or less whether it be 

 an abnormal rock or a normal igneous flow. 



^Idem, p. 81. 



83 See for example T. M. Chatard. Bull. 42, U. S. G. S., p. 45. 1887. 



™ Bull. 269, U. S. G. S., p. 94. 1906. 



71 K. Busz, Geol. Mag., p. 492. 1896. 



« A. K. CoomAra-Swamy, Quart. Jour. Geol. Soc, LVII, 185. 1901. 



73 L. V. Pirsson, "Petrography of the Igneous Rocks of the Little Belt Mountains," 

 Montana, 20th Ann. Rep. U. S. G. S., pt. 3, p. 554. 1900. 



74 A. Lacroix, "Sur l'origine du zircon et du corindon de la Haute Loire," Bull. Soc. 

 franc, de mineral., p. 100. 1890. 



