PAH-TSON MOUNTAINS. 777 



and friable texture, and is composed mainly of white quartz, red opaque 

 feldspar, and a silvery- white mica in thin laminae. 



These Archaean granites and schists are traversed by a number of interest- 

 ing dikes of a somewhat peculiar mineralogical composition. As to their age, 

 there is no definite clue, but they have been regarded as Archaean, partly from 

 their petrographical attitude in the field, and partly from the fact that they 

 were nowhere observed as penetrating the later granites. A dike in Crusoe 

 Canon consists of a fine pearl-gray rock, which serves as a kind of ground- 

 mass for a very coarse-grained pegniatite, in which are segregated a number 

 of accessory minerals. Colorless quartz occurs, both massive and fine- 

 grained. The orthoclase crystals frequently measure 3 and 4 inches in 

 length, of a brilliant lustre, apparently quite free from foreign ingredients. 

 The amount of soda present in these feldspars is quite small, as shown by 

 the following determination of the alkalies: Soda, 1.27 ; potassa, 13.45 per 

 cent. All the micas are light-colored, both muscovite, in transparent flakes 

 an inch or more in length, and thin laminae of lepidolite being present. 

 Tourmaline in long black needles, frequently well cr37^stallized, occurs in 

 segregated bunches, occasionally imbedded in quartz, when they seem to 

 arrange themselves in short narrow lines or bands. Grarnet, both massive 

 and well crystallized, is found, usually of a brownish-red color, intimately 

 associated with the colorless muscovite. No biotite or hornblende could be 

 detected in this rock, even under the microscope. 



Another dike penetrating the slates in the same region has a similar 

 composition, but is finer-grained, with the associated minerals somewhat 

 more disseminated through the mass. The tourmaline, especially, is so 

 abundant and so well distributed that the dike may be classed as a schorl- 

 granite. Running through the rock are bands of orthoclase, translucent 

 colorless quartz, and black tourmaline porphyritically enclosed. 



In the crystalline slates west of the summit of Pah-keah Peak occur 

 several dikes of fine-grained white granite, which traverse the older rock 

 in a northwest and southeast direction. They resemble the finer granite of 

 Crusoe Canon, and, if carefully examined, might be found to carry segre- 

 gations of the coarser material with all the accessory minerals. In these 



