390 



University of California. 



[Vol. 3. 



would apply to what might be termed a local habit of the orbules. 

 In other cases the concentric structure is more manifold, the 

 outer zone is prevailingly feldspathic and the radial structure of 

 the dark constituent is very pronounced. 



The finest specimen of this orbicular facies of the gabbro in 

 the possession of the writer is that sent him by Mr. H. P. Wood 

 of San Diego. Its exact location is not known, as it was evi- 

 dently a loose block taken from the slopes of the mountain and 

 weathered on all sides. This is made up of spheroids of about 

 six centimeters in diameter having a pronounced concentric and 

 radial structure. A photograph of this mass is shown in 

 Plate 46a. The spheroids are rather closely crowded together 

 and tend to fit together so that the spherical form is deformed by 

 a flattening at the points of abutment. Between the spheroids 

 the matrix is a medium coarse, granular, gabbro-like aggregate of 

 feldspar and a dark mineral. Occasionally, however, this inter- 

 orbicular matrix takes on the character of a granular aggregate 

 of feldspar in which the dark mineral is disposed in all azimuths 

 in the form of slender, often branching, rods. The structure of 

 the matrix in both conditions is shown in Plate 46b, which is a 

 photograph of a large thin section of one of the spheroids and 

 the adjoining matrix. 



Microscopic Characters . — A microscopic study of the rock in 

 thin sections brings out a number of facts which are not very 

 apparent on weathered or polished surfaces. At the center of 

 each spheroid there is characteristically a core consisting of an 

 alio triom orphic granular aggregate of basic plagioclase, probably 

 anorthite, and olivine with a subordinate amount of hypersthene. 

 This core has a diameter of from 12 to 25 mm. Commonly, 

 though perhaps not always, feldspar predominates at the center 

 of this core, and occasionally a single individual of feldspar may 

 be as much as 10 mm. across. This feldspathic portion grades 

 out toward the periphery of the core into an aggregate in which 

 the olivine is not less abundant than the feldspar, and in this 

 portion of the core there is often a more or less distinct radial 

 disposition of the olivine, but not nearly so well marked as in 

 the outer shells. 



