160 E. P. CAREY “AND W.J. MIEEER 
Chemical analysis.—Following is a partial analysis of the olivine 
gabbro furnished to Dr. J. C. Branner by the United States Geological 
Survey: 
NO eee ian, ii ike Went a TA Refeul Ci ene he Tate eed Rn OTE 
7 © eee e mathe PARR W dsm R Rear ee WRIT ume RAMA B IA eth! eTer} (Or 
Feg@ es Be @ es cai cai As eibeios ores Waren ke iad Sen ana a neal OIG 
TiO. Sa eM Cin aan earn Men oper DEE es GACH CaS 0 5 aan tKOL ICs 
CaO Bese aN ait Gail ah i ok Ale oe Ca A aN a er Te 
1h 6:4@ eee ecM NM me reat bit ginny Ee sib 50) 
Oe i ee Neat WARY Ce I fo af SRT ae ek eerie Mer ee Ce tee TV OTE 
Wotalhey ctu Seta ENS cee Ce tes Ae eno Ome 
THE OLIVINE PYROXENITE 
Occurrence.—This rock occurs mainly in and about the quarry, 
and is made up essentially of olivine and diallage with some magnetite. 
The percentage of diallage varies from a few scattered crystals, as in 
the massive varieties of serpentine, to a rock made up almost entirely 
of diallage. ‘The olivine is never wholly free from the process of 
serpentinization, except in the gabbro phase of the rock, and usually 
no feldspar is present. It is evidently a phase of the serpentine, the 
difference being chiefly in the relative proportion of the olivine and 
diallage. 
Petrographic characters.—A _ gneissic structure common to the 
gabbros is also, in this rock, very perfectly developed, and is accen- 
tuated at the surface by weathering. The diallage crystals are 
generally nearly a centimeter in length and quite uniform in size. 
Their cleavage surfaces are brassy in appearance, in some cases 
quite dark. When near the serpentine phase the whole rock becomes 
dark, resembling the massive serpentine. On the southeast side 
of the quarry entrance the rock is practically a diallagite, of a greenish- 
yellow color. On the south face of the quarry this rock contains 
feldspar and fresh olivine, thus passing into the gabbro phase. The 
optical properties of the diallage and olivine are the same as in the 
gabbro. 
THE PERIDOTITE-SERPENTINE 
Occurrence and associations.—Serpentine forms the greater part 
of the Oak Hill area and can be traced almost continuously severa 
