Fairbanks.] 



Ana/cite Diabase. 



279 



areas appear parallel partings, but the most have no structure and 

 are surrounded by a dark opaque border of thickly aggregated 

 magnetite particles. It would seem that there can be no doubt that 

 these products indicate the original presence of olivine. 



Magnetite is quite abundant as a primary constituent in the 

 form of grains and long, slender rods. A chemical test showed the 

 presence of titanium, while its magnetic properties distinguish it 

 from ilmenite. 



Analcite was evidently once quite abundant in the rock, filling 

 wedge-shaped areas between the feldspars and augite. It has 

 largely been replaced by the brown and green fibrous aggregates, 

 whose exact nature is indeterminable. The larger areas still show 

 in their centers an isotropic substance, which agrees in every par- 

 ticular with that proved to be analcite in the fresher dikes. The 

 manner of occurrence of the analcite, as well as the structure of the 

 rock, is shown in Plate 15, Fig. 1. 



MICROSCOPICAL PETROGRAPHY OF THE SECONDARY DIKES. 



Many sections were prepared from the large secondary dike 

 which had been opened by prospectors. This is by far the freshest 

 as well as most interesting of the secondary intrusions. The struc- 

 ture of the rock, as seen in thin section, is holocrystalline, approxi- 

 mately panidiomorphic. This dike, in common with all the second- 

 ary ones, contains no primary olivine; and the augite, instead of 

 following the feldspar in order of crystallization, has either crystal- 

 lized simultaneously or preceded it. In the latter case it exhibits 

 large idiomorphic crystals. 



Plagiocla.se. — The feldspar is present in the thin sections in short 

 lath-shaped and tabular forms. The characteristic twinning is that 

 according to the albite law, but in some instances it is found in 

 combination with the pericline law. All the individuals show 

 polysynthetic twinning in both broad and narrow plates, except the 

 broad tabular forms, which are evidently cut parallel to 00P00 , the 

 composition plane, as they show the emergence of a bisectrix with- 

 out the field of the microscope. The tabular forms exhibit a 

 remarkable zonal structure. Many individuals were observed with 

 a difference of io°, and several as much as 18 , in the angle of 



