THE TEREESTEIAL PERIDOTITES. — LHEEZOLITE. 129 



This isotropic character of the early stages of the alteration-products of minerals, has 

 been frequently observed by the present writer in the case of many other minerals. 



The sections herein described are of two slides, from Voigt and Hochgesang, purport- 

 ing to come from Vicdessos (European Collection, Nos. 71 and 165). Some additions 

 have also been made from the excellent description of Professor Bonney, to which the 

 student is referred.* 



This Iherzolite was regarded by Bonney as undoubtedly eruptive, on account of its 

 observed relations to the adjacent rock.f 



Scrrania dc Ronda, Spain. 



This rock, according to Macpherson, has a greenish groundmass of olivine, holding 

 emerald-green diopside. Under the microscope it is seen to be composed of olivine, 

 enstatite, and diallage (diopside). 



The diallage is of a clear green color, dichroic, and has a fibrous structure. The 

 olivine is clear and fissured, but shows in places a partial change to serpentine. The 

 enstatite resembles the diallage in its general cliaracters, but has a yellowish color. 

 Picotite is common. J 



IM>/. 



Numerous peridotites — Iherzolites and serpentines — have been described from Italy 

 b)- the Italian lithologists, particularly by Professors Alfonso Cossa and Torquato Tara- 

 melli. These appear to be composed principally of olivine, enstatite, diallage, and picotite, 

 and their secondary products. IMost of the serpentines seemed to have been formed by 

 the alteration of the Iherzolite variety of peridotite. Cossa's worlc contains many valu- 

 able chemical analyses of the olivine rocks which have been tabulated, and for the general 

 descriptions and plates the student is referred to Cossa's Eicerche Chimiche e Microsco- 

 piche su Eoccie e Mineral! d' Italia, Turin, 1881 ; and to the publications of the " Accade- 

 mia dei Lincei" of Eome. 



Ullcnihul, Ti/rol. 



A coarse, granular, greenish-white rock, according to Sandberger, holding bronzite, 

 cliromdiopside, and picotite, in grains and rounded octahedrons. A fine-grained variety 

 shows a schistose structui-e, and holds rose-red and deep blood-red pyrope. This rock is 

 altered in part to serpentine.§ 



Ridge lettveen Indian and Bear Vallejjs, Colusa Co., Cal. 



3001. A yellowish and grayish-brown groundmass, containing porphyritically 

 enclosed somewhat bronze-like crystals of enstatite and diallage. Under the lens the 

 groundmass shows a greenish network, holding a yellowisli or gray substance between the 

 meshes. Section : a greenish-white crystalline mixture of olivine, enstatite, and diallage. 



* Geol. Mag., 1S77 (2), iv. 59-04. 



f See also CharpentiL-r, Journal des Mines, 1S12, xxxii. 321-340 ; Essai sur la Constitntion Geogno- 

 stique des Pyrenees, 1S23, pp. 245-204. Ann. Physik, 1S14, hii. 201-20S ; Delamethcrie, Tlieorie de la 

 Terre, 1797 (2d eJ.), ii. 2S1, 282; Lefous dc Mineralogie, 1812, ii. 200, 207; Picot de Lapeyrouse, Mem. 

 Acad. Toulouse, iii. 410 ; Lelievre, Journal de Physique, 17S7, xxx. 397, 398 ; Vogel, Journal des Mines, 

 1813, xxxiv. 71-74 ; Zirkel, Zcit. Dent. geol. Geseli., 1807, six. 138-148 ; Damour, Bull. Soc. Geol. France, 

 1803 (2), xix. 413-410 ; J. Kiilin, Zclt. Dent. geol. Geseli., ISSl, xxsiii. 398. 



% Anal. Soc. Esp. Hist. Nat., 1S79, viii. 253-258. 



§ Neues Jahr. Min., 1S05, pp. 449, 450. 



17 



