CHICHAGOF COVE 87 



secondary. Pyroxene is very sparingly present, in colorless grains, and 

 is also much altered to chlorite. 



The rock is more feldspathic and certainly more alkaline than the 

 diorite-porphyrites which it cuts ; though it is here placed in the 

 same class with them, it may be more nearly related to the mon- 

 zonites. The chemical analysis necessary to determine this has not 

 been made. 



No. 112 is from a narrow dike on the shore of West Cove. It is 

 similar to the last, but somewhat less porphyritic, and has suffered 

 much alteration, so that the nature of the original bisilicate is in 

 doubt, although it appears to have been hornblende. The feldspars, 

 so far as they could be determined, are the same as in the preceding. 



Many other dikes of much the same appearance as NO. 82 were 

 seen, from which no specimens were collected. It is one of the 

 prominent types of dike rock in the region. 



OLIVINE-DIABASE AND DIABASE-PORPHYRITE 



A number of the dikes along the shore of West Cove proved to con- 

 sist of rocks more basic in character than those so far described, hav- 

 ing the composition and structure of diabase and diabase-porphyrite. 



No. 103 is the freshest and most characteristic of these rocks. It 

 is from a narrow dike, the first encountered in going northwest along 

 the shore from West Point. It is a heavy dark green rock, rather 

 coarsely granular, and traversed by many slickensided surfaces coated 

 with chlorite, which give it an appearance of advanced decomposition. 

 In the thin section it proves, however, to be much less altered than its 

 outward appearance would indicate. It is strikingly ophitic in struc- 

 ture, the principal constituents being lath-shaped crystals of basic 

 labradorite and large patches of violet-colored augite, allotriomorphic 

 to the feldspar. Other constituents are ilmenite and small amounts of 

 biotite. Secondary products are calcite and sericite, replacing wholly 

 or in part the feldspar ; chlorite derived from the augite ; abundant 

 bright green serpentine in patches whose form suggests a derivation 

 from olivine (a suggestion confirmed by the borders of magnetite 

 grains surrounding some of the patches) ; and leucoxene bordering or 

 wholly replacing the ilmenite crystals. The feldspar is cloudy with 

 decomposition products, but its extinctions are still distinct enough 

 to permit its determination. The augite is for the most part very 

 fresh, and only in one or two spots could it be seen to pass into chlo- 

 rite ; its color, taken with the presence of ilmenite in the slide, suggests 

 that the augite is titaniferous. 



