Lawson "I 



The Berkeley Hills. 



idiomorphic form. These crystals are much rounded and embayed 

 by magmatic corrosion, and fractured by mechanical strains. Its 

 relations to the other porphyritic constituents show it to be the 

 oldest secretion from the magma. The olivine is colorless and 

 non-pleochroic, with high relief, and strong double refraction. 

 Imperfect cleavage is indicated parallel to the pinacoids, the parting 

 parallel to the base being the most pronounced. The alteration of 

 olivine to serpentine may be studied in all stages of development. 

 It usually takes place along the nearly rectangular cleavage cracks, 

 along irregular curved cracks, generally present in the crystals, or 

 from the periphery of the crystal alone. The serpentine is for 

 the most part structureless, rarely showing a fibrous character, 

 the fibers at right angles to the surface of decomposition. As 

 alteration extends away from the cracks, the olivine substance is 

 generally divided up into isolated grains or "eyes," occupying the 

 center of a serpentine meshwork, and the ultimate product is a 

 serpentine pseudomorph, characterized by this mesh structure. 

 An exceptional mode of serpentinization was observed in one 

 large crystal of olivine. Alteration had begun at very numerous 

 points within the crystal and developed radially from them. No 

 definite arrangements of these points could be detected, though 

 the effect was that of an imperfect rectangular system. The round 

 spots of serpentine are connected by a system of extremely fine 

 curving cracks, each filled with serpentine; and in some parts 

 several adjacent spots have coalesced, forming a band of a second- 

 ary mineral. The aspect of the crystal thus affected was very 

 similar to that of the feldspar phenocrysts, with net-like arrange- 

 ment of augite or serpentine inclusions described elsewhere. 

 Olivine also appears to alter to a laminated yellowish mineral, with 

 characters which ally it to iddingsite. 



Magnetite is invariably present in the basalts as large aggregate 

 grains and in small sharply-bounded crystals, and elongated, rod- 

 like individuals. In one phase of the rock the sharp octahedral 

 crystals are in part scattered singly through the ground-mass, in 

 part grouped in beautiful reticulated aggregates, the bars of the 

 network formed by octahedrons placed point to point in parallel 

 position and branching at definite angles. Zones of grayish sub- 



G 



