THE TERRESTRIAL PERIDOTITES. — LHERZOLITE. 137 



olivine polarization. The secondary products are as in the preceding, but more abundant 

 and well marked. Much less enstatite and diallage e.xist here than in Xo. 65, and they 

 are more highly altered. The interspaces between the olivines more couiiuonly contain 

 pale greenish serpentine, bluish-green and yellovvi.sh biotite (?) material, dolomite, mag- 

 uetite, etc. Much of the serpentine shows the coarse fibrous laminae so commonly 

 observed in the serpentines described in this volume. In portions of this section and 

 in another slide from the same hand specimen the change of the rock mass to serpen- 

 tine is nearly, and sometimes quite complete, the forms of the olivines being distin- 

 guishable througli the arrangement of the magnetite and the serpentine. 



Tiie general structure of section is shown in figure 4, Plate VII. 



67, from the same locality, is composeil of the same dark grayish-black rock, which 

 here presents a brecciated appearance on account of its being traversed by rambling veins 

 of light-yellowisli serpentine. In the section the rock appears as a dirty greenisii mass 

 traversed by greenish-white veins. It is composed of serpentine pseudomorplis of 

 olivine, filled with beautiful dendritic growths of magnetite, as well as with irregular 

 masses of the same mineral. The interspaces contain the various secondary products 

 before described : micaceous (viriditic) material, serpentine, magnetite, feldspar, dolomite, 

 etc. The veins are composed principally of dolomite grains and serpentine, the former 

 predominating. 



74, from the same locality, is much like Xo. 65, but the grayish spots are more 

 abundant. Under the microscope it is seen to be composed of a coarsely fibrous lamellar 

 serpentine traversed by the irregular network of magnetite so common in the serpen- 

 tinized peridotites. It contains a little dolomite and one plate of a micaceous mineral 

 w^as observed. This was feebly dicluoic, greenish and yellowish, extinguished parallel 

 to the nicol diagonal, and polarized with a beautiful ])urplish-blue tiut. It presents a 

 fine micaceous cleavage parallel to the line of extinction, and it is probably partially 

 formed biotite. 



71, from the same locality, is a grayish-green rock traversed by grayish-white veins of 

 dolomite, which give to it a rude appearance of foliation. The rock contains a number 

 of reddish-browu patches formed from a breccia of the decomposed rock held by minute 

 reticulated veins of dolomite. The section is composed of yellowish and bluish green, 

 and reddish-brown pseudomorplis of olivine held in a granular mass of dolomite. 

 Crystals and grains of magnetite are scattered throughout the rock. The olivine in tlie 

 greenish pseudomorphs has been entirely replaced by serpentinous and ferruginous 

 material with dolomite, the first predominating. The reddi.sli-brown pseudomorphs have 

 the ferruginous material the most prominent, and it is the.se pseudomorphs which form 

 the reddish-brown patches before referred to. The fi.ssures of the olivine are represented 

 by ferruginous and sometimes by dolomitic bands, and these bands can sometimes be 

 seen in the dolomite, .showing the form of the pseudomorph when the latter has been 

 almost entirely replaced by dolomite. A portion of the structure of the section is shown 

 in figure 5, Plate A^IL, the gray groundmass being dolomite and the greenisii patches the 

 pseudomorphs after olivine. 



69, from the same locality, is a pale oil-green serpentine, banded and spotted with 

 dark and light brown. The section is composed of a granular and fibrous mass of 

 serpentine and dolomite, with ferruginous and earthy material, all holding brownish- 

 yellow earthy pseudomoi'iihs after olivine, which are traversed by the ferruginous bands 

 representing the olivine fissures. These pseudomorphs are wanting in portions of the 

 section. Much of the serpentine shows the fine fibrous polarization of chrysotile. 



18 



