1882.] 



143 [Julien. 



to the same volume ; by Dr. F. A. Genth of the University of 

 Pennsylvania, in his excellent papers on Corundum ; 1 and by R. W. 

 Raymond, in a short report on a survey of the Corundum veins near 

 Franklin. In the present brief sketch it is proposed merely to 

 offer some additional facts to which there has been little or no 

 reference, the general results of my study for several years, both 

 in the field and on a large collection of specimens and thin 

 sections. 



Form. As the beds are always highly tilted, they are seen 

 always in cross-section, and their tracts generally present irregu- 

 larly oval or elliptical outlines, or, in the smaller masses, those of 

 decidedly lenticular layers. The major axis of such a mass 

 reaches the length of about 1.5 kilometers in the largest bed, 

 that of Cullakenee (Buck Creek) in Clay County, and the width 

 of about 200 meters, the creek having cut its way through the 

 length of the deposit and affording a good section of the layers 

 upon its bare, sloping banks. 



Lamination. The rock always possesses a marked slaty lami- 

 nation, exactly like that of the slaty hornblende-gneisses sur- 

 rounding it, the distinct laminae usually varying from i to 1 

 cm. That these laminae really indicate the bedding planes of a 

 mechanical sediment, and not the characteristics of a chemical 

 deposit, is showm by three facts, — 



1st. On microscopic examination of thin sections, transverse to 

 the lamination, there is always shown an alternation of coarser and 

 finer irregular grams, the certain mark of a sorting out of sedi- 

 ments deposited in water. 



2d. The chromite-grains are not only dispersed through the 

 dunyte, in exactly the same way as those of iron-ore through the 

 siliceous sands on a sea-coast; but are often concentrated in 

 laminae about a centimeter in thickness, alternating with those of 

 olivine, or even in coarser layers of a chromite-breccia, with 

 kaemmererite-scales acting as a cement. These coarser layers are 

 often spoken of as "veins," but always lie in the plane of stratifi- 

 cation, and often show the sorting process among their own 

 grains. 



3d. At a few localities, near the margin of a huge mass of 

 dunyte, this rock is found to be interbedded with the hornblende- 

 i Am. Phil. Soc, Sept. 19, 1873, and July 17, 1874. 



