220 R. VAN RISE — Pl.T-i \Ml:l;l\\ OF THE B] VCK HILLS. 



lively larger proportion of purely clayey materials. They also <•< ► 1 1 1 :i i n in 

 many cases a very large proportion ofpyrite mingled with theferrite. The 

 total of these ferriferous materials is much greater than in any of the coarser- 

 grained micaceous rnrk-. Newton states that the black slates contain car- 

 bonaceous material. It' this is 1 1 1 « - case, and I have no doubt that it is, the 

 pree of the large amount of iron sulphide may be explained by the 



reducing action of such organic matter. 



In a few cases have been found, in the less thoroughly crystalline grey- 

 wackes. a class of r""k in which the alteration product of the detrital feld- 

 Bpar i- a variety of amphibole. In (his decomposition the relations between 

 the feldspar and amphibole are exactly like those between the feldspar and 

 mica above described. The amphibolitic greywackes differ macroscopically 



fr the mica sjreywackes only in that they arc greenish grey rather 



than grey. I -i the possibility that the decomposition of a 



quartz-feldspar fragmental rock may also under favorable circumstani 

 produce a hornblende-schist : but while hornblende-schists are found inti- 

 mately associated with the micaceous slates and schists, the connection be- 

 tween them and these greywackes, it' any exists, has not been worked out. 



/ Mica-gneisses. — In most of the mica-schists the <• litions have been 



such that the old fragmental feldspars have decomposed. In a few of them 

 an- found small individuals of perfectly fresh feldspar which inclose particles 

 <>t' secondary ferrite. From their appearance they are taken, like the mica. 

 to be a new crystallization. Ajb was first described by Teall,* and subs 

 quently by other writer.-, old feldspars have here broken down, and at the 

 same time new ones have formed of a different character. In a lew cat 



amoug the most crystallii I these mica-schists the amount of such original 



feldspar is sufficient to make the rocks a muscovite-biotite-gneiss. In their 

 macroscopic appearance these rocks do not differ from the mica-schists just 

 ilescribi I. In tbiu sections, their background, instead of consisting almost 

 wholly of quartz, as in the mica-schists, is of evenly granular quartz and 

 feldspar in approximately equal amounts. The feldspars are for the most 

 part perfectly fresh, and comprise orthoclase, microcline, and plagioclase. 

 'The latter i- twinned both according to the albite and pericline laws, both 

 kinds of twinning often being found in the same individual. The inter- 

 iking of this quartz-feldspar background is as intricate as in any ordinary 

 of the same di >f coarseness. Both mica- are abun 



dantly present, hiotite being the more plentiful. The Kind.- are arranged 

 in mon direction to a considerable extent. In one case the twin 



lamella; of the plagiocla erally correspond in direction to the folia of 



mica and the tion of the grains of quartz. In th< of microcline 



and the double twinned pla i the twin lamella; of course run in this 



i il I'eall 



