Ransome.) 



Eruptive Rocks of Point Bonita. 



IOI 



gates, some comparatively close and massive, with a bronze-brown 

 color, others in more open, delicate, colorless forms. Both classes 

 of aggregates appear to have developed independently of the rods, 

 which intersect the brushes in different directions. Numerous 

 cracks, filled with some colorless mineral, traverse the varioles 

 irregularly, and when they are narrow and straight are not dis- 

 tinguishable from the rods* Under crossed nicols the different 

 brushes and groups show partial and distorted black crosses, while 

 the rods and cracks polarize irregularly like aggregates. With 

 high powers the rods appear as spaces filled with some transparent 

 material into which project the ends of microlites on either side. 

 This material does not effervesce with acids, and is probably feld- 

 spathic, the rods corresponding to the pseudo-crystallites of M. 

 Michel Levy.f 



The microlites composing the dense brown brushes could not 

 be separated distinctly with high powers. With crossed nicols they 

 give suggestions of bright polarization colors, and extinguish at 

 a considerable angle, apparently about 40° They are probably 

 augite. 



The light gray groundmass, in which the varioles are imbedded, 

 is seen under the microscope to be made up of a mass of delicate 

 arborescent growths suggesting the hoarfrost on a window, or a 

 forest of minute conifers prostrated by the wind. A few of the 

 clusters radiate from a central point, others from a short line, but 

 by far the most common arrangement is that of a central rib, to 

 which the microlites stand perpendicular on either side, recalling 

 no image so suggestive as a young symmetrical fir-tree. With 

 high power, the microlites appear as little transparent colorless rods 

 with blunt, rounded ends, showing parallel extinction and blue-gray 

 interference colors. They are doubtless feldspars. They exhibit 

 none of the " cone-in-cone " structure as described by Cole, J but, 



*J, W. Gregory, The Variolitic Diabase of the Fichtelgebirge, Q. J. G. S.. 

 Vol. XLVII, p. 56. 



fMemoire sur la Variolite de la Durance," Bull. Soc. G6o\. France, 3™" 

 Ser., Vol. V, p. 238. 



See also Delesse, Sur la Variolite de la Durance, toe. cit., p. 118, with cut. 



JOn Some Examples of Cone-in-Cone Structure, Min. Mag., Vol- X, No. 

 46, p. 136, 1893. 



