210 



University of California . 



[Vol. i. 



jected to explosions and attrition within one or more of the vents 

 through which portions of the fourchite magma found their way up 

 from below. 



Mention has already been made of the interesting nature of the 

 igneous intrusion at Point Blunt, and the interest is not diminished 

 when the eruptive rock is studied under the microscope. A speci- 

 men taken near the southwestern corner of the point does not differ 

 at first glance from the fourchite of the sill, but the lens shows that 

 it contains small lath-shaped sections of feldspar. Under the 

 microscope it might be classed as an ordinary diabase. The lath- 

 shaped plagioclases predominate in well-developed crystals, showing 

 twin lamellae, while the augite is rather subordinate in amount. 

 The latter is inclined to be granular in form, and is identical with 

 the augite of the sill. Magnetite is abundant, and there is also con- 

 siderable chlorite and calcite present. The rock has a specific grav- 

 ity of 2.97. This fades may be regarded as a connecting link 

 between the fourchite as it occurs in the sill, and the rock next to be 

 described. 



The spheroidal eruptive, previously referred to as forming a con- 

 spicuous stack just off the point, and accessible at low water, is a 

 dense, greenish rock, showing no distinct macroscopic crystals. In 

 thin section (PI. 13, Fig. 2) it affords the most beautiful example 

 yet seen, of the brush-like grouping of slender plagioclase crystals 

 which was described in a former paper as characteristic of simi- 

 lar dense green eruptive rocks with spheroidal structure, occur- 

 ring at Point Bonita, Mount Diablo, and elsewhere. The plagioclase 

 crystals take on various forms. Sometimes they are slender needles 

 sharply pointed at both ends; sometimes lath-shaped with frayed 

 and bristling ends. Delicate skeleton crystals, consisting of two 

 long parallel needles connected at their middle points, are abundant. 

 Many of the slender crystals show in perfection the beautiful brush- 

 like wisps described in the paper referred to. With nicols crossed, 

 it is seen that the feldspar microlites and crystals are largely decom- 

 posed, but enough of the original substance remains to show their 

 feldspathic character. Some of the broader, lath-shaped sections 

 show distinct twinning lamella;. Certain brown brushes, whose 

 fibers extinguish at large angles, and show fairly bright polarization 



