ECLOGITRS IN CALIFORNIA 353 



The garnets in the hornblende eclogite have a fresh, clear color, 

 and show little sign of change, but they are much cracked. Small 

 quantities of feldspar and of quartz are found in the different slides. 

 An opaque irregular mass shows by reflected light a yellow center and 

 a black border. It is undoubtedly pyrite surrounded by iron oxide. 



A hand specimen of eclogite has recently been sent to the writer 

 by W. D. Smith, who collected it at a point some four miles south of 

 east from the Calaveras exposure described in this paper. It carries 

 coarse red garnets up to a half-inch in diameter, some of them showing 

 well the rhombic dodecahedral form. The groundmass seems to be 

 chlorite, glaucophane, and omphacite. The eclogite most resembles 

 the San Martin variety, but is not quite so fresh in appearance. No 

 igneous rock is reported from the vicinity. 



Tihuron. — In addition to the interest attached to this locality 

 because of the discovery of lawsonite, the peninsula of Tiburon in 

 San Francisco Bay is noteworthy from the number of outcrops of eclo- 

 gite found there. Much of the rocky knoll near Reecl Station, where 

 the lawsonite occurs, is eclogite of the glaucophane-omphacite 

 variety. The minerals in this rock have been described by Ransome.^ 

 On the top of the hill above the lawsonite are two outcrops of eclogite 

 occurring in the serpentine area. These have more omphacite than 

 glaucophane, while on the slope of the hill beyond a glaucophane 

 eclogite occurs that is almost free from omphacite. The slides from 

 the first outcrop were mainly selected to show lawsonite, and so the 

 proportion of that mineral appears unduly large. The matrix is 

 usually composed of small allotromorphic crystals of a grass-green 

 color intergrown with glaucophane. The green mineral shows no 

 pleochroism in most cases. The extinction is frequently wavy, but 

 where it admits of measurement the maximum angle is close to 40°. 

 The mineral is identified as omphacite, with a probability that a 

 portion of the green matrix is hornblende. In some cases the garnets 

 are almost entirely replaced by chlorite; in others there is a celyphite 

 border of the chlorite. In one instance the garnet has a celyphite 

 border of mica. Mica is very common in these slides, and is deter- 

 mined as margasite by Ransome, to whom reference is made for the 

 full description of the associated minerals. 



I "On Lawsonite, a New Rock-Forming Mineral," Bulletin of the Department of 

 Geology, University of California, Vol. I, p. 311. 



