Ransomk. ] 



Eruptive Rocks of Point Bonita, 



95 



length and only about I mm. in width, while the other minerals 

 show similar elongated forms. Under the microscope (Fig. 8) the 



Figure 8. — Section of diabase near lighthouse, with large mass of ilmenite 

 (and magnetite), a, light greenish augite ; a, light pinkish 

 augite ; p, plagioclase ; c, chlorite. • 35. 



rock shows a coarsely crystalline ophitic structure, in which occur 

 irregular patches of a dense green groundmass in which little can 

 be made out except remnants of lath-shaped feldspars, grains of 

 opaque iron ores, phenocrysts of plagioclase, and much secondary 

 quartz, chlorite, and light green hornblende. The ophitic portion 

 of the rock consists of much decomposed idiomorphic plagioclases, 

 augite in large irregular plates, and opaque iron ores ranging in 

 size from small grains up to aggregate 7 mm. in length, and large 

 masses with the form and appearance of ilmenite (Fig. 8). The 

 augite is colorless to light greenish or pinkish shades, and with 

 little or no pleochroism. It shows more signs of alteration than 

 usual, and is sometimes bordered by little plates and tufts of light 

 green secondary hornblende, which penetrate it irregularly and 

 grow as if rooted in its surface. Apatite is abundant, in long 

 slender prisms, which pass uninterruptedly through plagioclase, 

 augite, and the dense groundmass, indicating that the rock solidi- 

 fied quietly in situ. That this rock is of the nature of a local mod- 



