GABBKO. 251 



from mottled feldspars 3 mm. long, which are poikilitic. There are no 

 phenociysts. In thin section it consists of small, stout idiomorphic prisms 

 of plagioclase which have the optical characters of labradorite, besides stout 

 prisms of augite and hypersthene, partly idiomorphic, together with magnetite 

 and a very small amount of biotite. These minerals are scattered irregu- 

 larly in broad allotriomorphic individuals of orthoclase, which act as a 

 cementing matrix, but do not equal the plagioclase in amount. It is nearly 

 a monzonite. There is a very little quartz. Its structure is shown in PI. 

 XXXIV, fig. 1. Apatite occurs in stout and in slender prisms. The 

 hypersthene is very slightly altered in places ; otherwise the rock is 

 extremely fresh. The small amount of biotite is specially noteworthy 

 when the chemical composition of the rock is compared with that of 1396 

 and 1399 (analyses 8 and 7 on page 260), both of which are rich in biotite. 



Returning to the gabbro core, Ave have to consider a number of intru- 

 sions within the main body of granular gabbro and in the aphanitic 

 marginal zone which form dikes representing finer-grained and porphyritic 

 modifications of the gabbro magma. The size and distribution of the 

 phenociysts of feldspar and pyroxene show their relation to the rocks of 

 the outlying dikes with porphyritical feldspars and pyroxene, as well as to 

 those of the breccias and flows, while their microstructure connects them 

 with the gabbro. They present intermediate phases of crystallization 

 between the two, and if followed continuously into the surrounding country 

 would connect the granular gabbro with the basalt. 



Of these intrusions or dikes one (1398) occurs on the west spur, at 

 8,650 feet altitude, cutting the granular gabbro. In thin section it consists 

 of labradorite with irregular outlines in a groundmas's of rounded crystals 

 of feldspar, augite, hypersthene, and magnetite, with some biotite and 

 orthoclase and a little quartz. The structure is similar to that of 1383, but 

 is coarser grained. The phenociysts of labradorite, augite, and hyper- 

 sthene have the inclusions characteristic of the gabbro-porphyries. 



Another dike of this rock (1413) cuts the gabbro near the outlet of 

 the lake. The dike is 6 inches wide, and the rock is dark gray and crys- 

 talline, with the same microscopical habit as the one just described. The 

 microstructure is intermediate between the latter and 1388. The feldspars 

 are dusted at the center, the outer portion carrying many small crystals of 

 biotite, which is specially abundant in this rock. The third occurrence is 



