TIIK TKL'KKSTHIAL PKUI DOTITES. — LII KKZOI.ITK. ].',] 



rocks by an e.\uiiiinati(ni of their relations to the associat^'d rocks. Of the occurrence 

 Mr. Goodyear states : " The great mass of the rock throughout the wliole ridge consists 

 of, apparently, a serpeutinoid matrix, filled with foliated crystals of a hard, green mineral, 

 which I suspect to be pyroxene, forming a rock similar to that of which large quantities 

 occur near Guenoc and Coyote Valley. But there are also immense quantities of seq»en- 

 tine without these crystals."* 



The associated rocks, according to Mr. Goodyear, were some hard metamorphic sand- 

 stones, and a few shales. 



No. 3002 is from the same locality. 



This has a reddish-brown groundmass, holding crystals of enstatite and diallage. The 

 same reticulated network is observed in the groundmass as in the preceding, but the in- 

 closed portions are of a yellowish- or reddish-brown color. A rouglily banded appearance is 

 produced by a somewhat linear arrangement of the enclosed crystals. Both this and the 

 preceding are surface specimens. Some of the crystals in X(j. 3002 show the well- 

 marked characters of bronzite. Section : this is composed of a reticulated network of 

 serpentine veins, holding rounded and irregular grains of olivine, enstatite, and diallage ; 

 while larger enstatite crystals are porphyritically enclosed. This rock was evidently once 

 a crystalline-granular mass of olivine, enstatite, and diallage, but now it exhibits a stage 

 of alteration somewhat in ailvance of that shown in No. 3001. The same reticulated 

 network of serpentine, with the same reddish-brown medial line, is to be observed as in 

 the preceding ; in fact, the structure of the two rocks is identical. The serpentine ex- 

 tends from the medial line of the veins inward along the fissures, until only portions 

 of the original minerals are left surrounded by it. In many cases the serpentine has re- 

 placed the entire mass of the rock, but still retains the marks of the fissures along 

 which the alteration took place. The serpentine extending out from the reddish-brown 

 portion of the veins is of a pale greenish-yellow color, and shows fibrous and aggregate 

 polarization — the fibres, as usual, being perpendicular to the sides of the vein. While, 

 part of the olivine lying inclosed in this network is unchanged, much of it has been 

 altered to a reddish-brown serpentinous mass like that forming the centre of the veins. 

 In many cases this last extends only partly through the olivine grain leaving a central 

 portion of unchanged olivine, but in others the alteration is complete. This reddish- 

 brown alteration shows a tendency to extend in fibres parallel to the crystallographic axis, 

 or along the latent cleavage planes. The general characters given in describing the minerals 

 in No. 3001 hold good here. The enstatite is more highly altered to the greenish fibrou.s 

 product, while it is frequently crossed by fissures at right angles to the principal cleavage. 

 The serpentine veins in this and in the diallage are more abundant and pronounc.-'d than 

 in No. 3001 ; but these minerals evidently are more slowly altered than the olivine. Of 

 the enstatite and diallage, the former is the more readily changed. Picotite or chromite 

 occur as before, but in somewhat larger masses. 



Figure 2, Plate V., shows the general microscopic structure of this rock, the ilark to 

 black portions representing the iron-ore grains ; the white portions are the unchanged oli- 

 vine, the orange-brown and yellow colors mark the differently altered portions of the 

 olivine (serpentine), while the dark-brown bands are serpentine veins like those which 

 are shown to some extent in figure 1 of this plate. 



Figure 3, Plate V., is from the same rock, and shows the structure of the partly- 

 altered enstatite. The main portion of the figure is that mineral \vith its cleavage linos 



* Uiipublibhecl Kcpoi't mado to ProiV.-isor J. 1) Wliitiioy. 



