THE TERRESTRIAL rEIIIDOTITES.— LHERZOLITE. 137 



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

 and well marked. Much less enstatite and diallage exist here than in Xo. 65 and thrv 

 are more highly altered. Tlie interspaces Ixitween the olivines more comtnoidy contain 

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

 netite, etc. Mucli of the serpentine shows the coarse fibrous lamiuic so commonly 

 observed in tlie 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 bein" distin- 

 guishable through the arrangement of the magnetite and the serpentine. 



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



67, from the same locality, is composed of the same dark grayish-ldack rock, wbich 

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

 of light-yellowish serpentine. In the section the rock appears as a dirty greenish mass 

 traversed by greenish-white veins. It is composed of serpentine pseudomorphs 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 No. 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 

 was observed. This was feebly dichroic, greenish and yellowish, extinguished paralk-l 

 to the nicol diagonal, and polarized with a beautiful purplish-blue tint. 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-brown 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 pseudomorphs of olivine held in a granular mass of dolomite. 

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

 greenish pseudomorphs has been entirely replaced by serpentinous and ferruginous 

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

 the ferruginous material the most prominent, and it is these pseudomorphs whi'^h form 

 the reddish-brown patches before referred to. The fissures 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 pseudomorpli Avhen the latter has been 

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

 in figure 5, Plate VII., the gray groundmass being dolomite and the greenish patches the 

 pseudomorphs after olivine. 



69, from the same locality, i§ 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 pseudomorphs after olivine, which are traversed by the ferruginous bantls 

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

 section. Much of the serpentine shows the fine fibrous i)olarization of chrysotile. 



IS 



