40 HAEHL AND ARXOLD — THE MIOCENE DIABASE. [Feb. 5, 



analcite from the andesites in the Berkeley Hills. At several points 

 in the area small irregular aggregates, varying in size from one-tenth 

 to one-half of an inch in diameter, made up of fan-shaped growths 

 of slightly clouded, white crystals, were found in the weathered dia- 

 base. When tested before the blowpipe these crystals were found 

 to be natrolite, and a thin section cut from one of the small bodies 

 showed the angular centre area between the natrolite crystals to be 

 filled with analcite (see Fig. 25). Calcite veins of considerable 

 size were found in the mass in sume places. 



Macroscopically the rock is fine grained and dark. Augite and 

 olivine crystal are readily detected in most specimens, sometimes 

 in crystals large enough to be porphyritic. Plagioclase feldspar 

 and magnetite are also present, and pyrite has been found in a few 

 places. The augite is dark and lustrous and usually quite fresh. The 

 olivine, however, is generally somewhat weathered to serpentine, 

 which often fills the crystal cavity completely and gives the rock a 

 greenish tint. Another weathering jDroduct of the olivine was 

 found quite plentifully in thin scales of light brown color. Chem- 

 ical tests showed the presence of Na, Ca, Fe and Mg. The mineral is 

 hydrous and infusible. Treated with hydrochloric acid, it becomes 

 lighter in color and gives up its iron. These, together with its op- 

 tical properties, which will be mentioned, make it possible that it 

 is the mineral described by Lawson^ as iddingsite. The feldspars in 

 this facies are almost universally microlitic. An occasional pheno- 

 cryst is seen. Magnetite is present in small grains, barely visible 

 to the unaided eye. The basaltic facies usually has a very distinct, 

 coarse, conchoidal fracture. Figure i8« is a photograph of speci- 

 men No. ^S, a piece of the typical basaltic facies, the analysis 

 of which is given as II on a following page. 



Microscopic Petrography. 



The petrographic discussion contained herein is based upon the 

 study of about one hundred and thirty slides, cut from the rocks col- 

 lected over the diabase area and examined under the microscope. 

 In thin section the eruptive presents two facies. Both are holo- 

 crystalline and contain about the same minerals, but the one pre- 

 sents a rather granular structure under the microscope, just as it 

 does in the hand specimen; the other a finely crystalline, aphanitic 



i"The Geology of Carmdo Bay," Bu//. Dept. Geol. Univ. Cal., Vol. I, p. 31. 



