Lawson.] 



Orbicular Gabbro. 



389 



The Orbicular Fades. — The orbicular facies of the gabbro 

 was found by the writer on the south side of the mountain near 

 the western boundary of the gabbro mass. The orbicular struc- 

 ture appears prominently upon the weathered surface of the 

 gabbro, but the portion of the mass which is affected by this 

 orbicular structure has no sharp boundary against the normal 

 facies of the rock. It rather grades into it insensibly by an 

 increase in the intervals between the orbules and by a feebler 

 development of the latter till eventually they become in the 

 weathered surface indistinguishable from the rest of the rock. 

 Where the orbicular structure is well developed it is apparent on 

 the weathered surface of the i*ock by reason of the differential 

 weathering of the light and dark constituents as well as by their 

 difference of color. The feldspar has been etched more deeply 

 than the ferro-magnesian constituents, and the latter, therefore, 

 stand out in relief. The sections of the orbules afforded by rock 

 surfaces are in nearly all cases approximately circular, thus 

 indicating the absence of any marked deviation from the spheri- 

 cal form. The limits of the orbules against the rest of the rock 

 are usually sharp, and within these limits there are two pro- 

 nounced structures which give the orbules their character. One 

 of these is a concentric disposition of the light and dark con- 

 stituents, and the second is the radial disposition of the dark 

 constituents. The first of these is the more striking and on the 

 rock surfaces gives rise to concentric circular zones of alternating 

 light and dark material, the latter being in relief. A very 

 common arrangement is one in which the circular sections of the 

 orbules present a light colored central area, having a diameter of 

 about one-third the full diameter of the orbule, and composed 

 evidently chiefly of feldspar. Encircling this is a broad zone of 

 dark material in relief, evidently some ferro-magnesian mineral 

 for the most part, although with an admixture of feldspar. Out- 

 side of this is a narrow zone almost entirely feldspathic and 

 encasing this is a zone, usually very narrow and sharp, composed 

 of the dark constituent ; or there may be two of these very narrow 

 sharp dark zones with an intervening light zone also very narrow. 

 The outermost shell of the orbule appears generally to be repre- 

 sented by this narrow zone of dark mineral. This description 



