120 PERIDOTITE. 



In the second case, the alteration consists in the formation of a few crystals, and in 

 every gradation from that to a state in which theduuite has been transformed into a more 

 or less schistose roclv, largel}^ composed of hornblende, and actinolite or treniolite. A few 

 grains of olivine usnall}'' remain unchanged even in these extreme alterations. 



The third change is brouglit about either by the direct alteration of the olivine, or by 

 the conversion of the secondary actinolite itself into talc. Through this alteration talcose 

 rocks are formed, like talc-schists ; as well as amphibolitic or olivine ones bearing talc. 



The fourth alteration has been descril>ed by me in the preceding account of the Web- 

 ster peridotite, and hence I will not here quote from Dr. Julien, iarther than to say that 

 according to him talc is frequently associated with the serpentine, thus forming a talcose 

 serpentine. 



The fifth and last alteration is "confined to a single locality, and consists of an inter- 

 nal conversion of the olivine into ampliibole — a bright grass-green variety which Dr. 

 Genth has identified as sniaragdite or kokscharoffite — and albite, sometimes witli abun- 

 dantly disseminated particles of ruby red corundum, producing a peculiar variety of 

 diorite or gabbro. Again, this very rock has been subsequently attacked by a secondary 

 process of alteration, the albite grains being enveloped by an alteration-crust of margarite, 

 and the condition of hornblende modified. Tiie result of this action is a coarse margaritic 

 gabbro." 



Dr. Julien believes that many of the amphibole and tnlc-bearing schists and serpen- 

 tines along the Appalachian belt are the equivalents of the North Carolina dunite. 



The North Carolina peridotites have been described in previous papers by Genth, 

 Jenks, Kerr, C. D. Smith, Shepard, J. L. Smith, Eaymond, and others.* 



In most of the above papers, corundum is especially treated of, since it has been 

 largely found associated with the peridotites of the Southern States. This mineral Genth 

 regards as original, but Julien as a secondary product of alteration. Various opinions 

 have been advanced concerning the North Carolina peridotite — that it is of chenncal, 

 sedimentary, and eruptive origin. Messrs. Kerr and C. D. Smitli who have, except Julien, 

 studied the rock most in tlie field, regard it as eruptive, but the published evidence given l)y 

 them is, like Julien's, not conclusive. The reasons that the present writer has for believing 

 this rock to be of eruptive origin have already been given. It hardly seems possible that 

 the olivine could liave been deposited as a loose sand, exposed to water and air, consoli- 

 dated, and remained until the present time unchanged.! 



Tafjord, Nonmy. 



The rock from Tafjord, Norway, as seen in a section purchased from Eichard Fuess, 

 Berlin, is composed principally of rounded grains of olivine, with some enstatite, cof- 

 fee-brown picotite or chromite, and a little magnetite. The structure is essentially the 

 same as that of the peridotite from Franklin, North Carolina. The form and arrangement 

 of tlie enstatite are very similar to those of the talc in No. 5134, 



Mohl describes this rock as being similar to the one from Eodfjeld, but with less 

 enstatite, and some magnetite. \ 



* Am. Phil. Soc. Proc, 1873, xiii. 3C1-406 ; 1874, xiv. ; 1SS2, 210-218, 381-404; Quart. Jour. Geol. 

 Snc, 1874, XXX. 303-306 ; Geol. of North Carolina, 1875, vol. i. 129-130 ; Ai)|)cmlix D, pp. 91-97, 102- 

 107 ; 1881, vol. ii. 42, 43; Am. Jour. Sci., 1872 (3), iii. 301, 302; iv. 109-11.5, 175-180; 1873, vi. 180- 

 ISG ; Pop. Sci. Monthly, 1874, iv. 452-456 ; Traus. Am. lust. Mill. Eng., 1878, vii. 83-90. 



t Proc. Bost. Soc. Nat. Hist., 1882, xxu. 141-149. See also Science, 1884, iii. 486, 487- 



+ Nyt Mag., 1877, xxiii. 115, 116. 



