Faikranks.] 



A nalcite Diabase. 



which fell in the first trial. This gave a maximum specific gravity 

 of 2.571, the powder in both cases appearing alike under the 

 microscope. There was no point at which any large proportion of 

 the feldspar came down. It fell little by little with increased dilu- 

 tion. The results obtained are so far below the specific gravity of 

 any of the soda-lime feldspars that they can only be attributed to 

 the dissemination of exceedingly minute particles of analcite 

 through every portion. 



Under the microscope the feldspars appear to be undergoing 

 decomposition to several distinct products. The most prominent 

 of these is a clear isotropic substance, identical in appearance with 

 the analcite filling the angular spaces, even showing the same opti- 

 cal anomalies. In many instances this alteration has spread over 

 nearly the whole crystal. In the case of some it has begun on the 

 outside, in others from the center, leaving a mere shell. It is com- 

 mon to find that those crystals which contain much analcite are 

 less altered to the other secondary products, such as kaolin and the 

 green granular products. The analcite differs from that in the 

 angular spaces between the crystals in being perfectly fresh. The 

 analcite is often distributed through the crystals in angular sharply- 

 defined areas, giving the appearance of a mosaic of the cuneiform 

 characters of the quartz penetrations in graphic granite. (Plate 15, 

 Fig. 2.) In these cases the remaining portions of the feldspar are 

 perfectly clear and polarize brightly. When even but a small 

 portion of the feldspar is left, it is perfectly clear. Those portions 

 of the feldspars which join the angular areas of analcite, where the 

 latter are more or less replaced by a green fibrous aggregate, are 

 also invaded by the same along cracks and cleavage planes, and 

 in extreme cases are almost wholly replaced by it. Decay of 

 the feldspars to kaolin is also widespread. Sections made from 

 many other dikes show much the same manner of decomposition 

 for the feldspar. In every section the latter is more or less 

 replaced by analcite, but kaolinic and green products are more 

 characteristic. 



Lamellar Pyroxene. — The pyroxenic component appears to be 

 augite, but it exhibits some peculiar phenomena allving it to dial- 

 lage. As distinguished from the orthorhombic pyroxenes, it gives 



