492 C. Palache — Occurrence of Olivine. 



The specimens of olivine discovered by Mr. Cowles occurred 

 in the railroad cut where it passes through this same Middle- 

 field serpentine mass, probably on the Chester side of the town 

 line. He noticed the mineral, which he took for apatite, in a 

 narrow vein on the side of the cut, exposed during the widening 

 of the road b'ed ; collecting specimens at the time, he also noted 

 the spot beyond the cut where the rock was being dumped by 

 the workmen, and it was from blocks thus located, several hun- 

 dred yards from the place of occurrence, that we were able to 

 collect material. The olivine forms a narrow vein, two inches 

 or less in width, cutting massive serpentine like that described 

 above. The olivine is dark to light green in color, vitreous in 

 appearance and hard; parts of the vein are completely filled by 

 granular olivine ; other parts of the vein show the olivine in rude, 

 rounded crystals, up to an inch in length, embedded in a matrix 

 consisting of greenish white serpentine with the structure of 

 picrolite, densely felted white chrysotile and occasionally 

 broad plates of clear cleavable brucite. Large anhedra of mag- 

 netite up to an inch across occur rarely in the vein and, like the 

 olivine crystals, these are wrapped around by the matrix in in- 

 timate fashion. Although some of the olivine crystals have 

 undergone partial serpentinization and show glistening scales 

 of brucite, it is easy to find those which appear perfectly fresh 

 and show under the microscope the characteristic appearance 

 and optical characters of the mineral. 



Such material, carefully selected as free from visible impurities 

 as possible, was analyzed in the laboratory of the IT. S. Geolog- 

 ical Survey by Mr. Schaller and the analysis is published here by 

 permission of the Director and through the kindness of Profes- 

 sor Emerson, for whose studies it was made. 



Si0 2 39-43$ Note by Mr. Schaller. "Some 



FeO 7*83 impurity from the grinding ma- 



MgO 49-26 chine was accidentally introduced 



MnO ... -12 into the sample. The value for 



CaO none ferrous irou represents total iron." 



F none 



H„0- 1-20 



h;o+ i-49 



CO -11 



100-10 



The analysis shows that the material was less fresh than it 

 appeared. But if C0 2 be regarded as present in form of mag- 

 nesite and H 2 as equally divided between brucite and serpentine, 

 both known to be present in the sample, we have 



