PETROGEAPHICAL OHARACTEES OF UPPER HUEONIAN. 169 



the siderite to magnetite, and produced from the fine fragmental mud tlie 

 muscovite and chloriie. 



MICBOSCOPICAL DESCBIPTION OF CERTAIN OF THE SEDIMENTAEIES. 



In the following pages I shall describe in a brief way the graywackes 

 and slates, the most common rocks of the district, and the rocks which 

 have been produced from them by metamorphism. 



The graywackes and slates consist chiefly of readily distinguishable 

 fragmental quartz and feldspar grains, which , are embedded in a matrix 

 consisting of fine-grained quartz, feldspar (!), biotite, muscovite, chlorite, 

 some siderite, epidote, small quantities of magnetite, hematite, and iron 

 pyi'ites, and a dark clayey mass. This mass appears to contain a consider- 

 able amount of black carbonaceous material and reddish-brown ferruginous 

 matter in finely disseminated specks. The greater the quantit)- and finer 

 the character of this matrix the more difficult it becomes to determine its 

 constituents with any degree of certainty. In the slates the matrix plays 

 the chief role, while in the graywackes the large fragmental grains form 

 the predominant material. By a diminution in quantity of the matrix and 

 fragmental feldspar grains, the coarser-grained elastics approach very closely 

 to true quartzites, but in no case was a pure quartzite found. 



T'he constituents which can be recognized without difficulty as original 

 ones are the larger grains of feldspar and quartz. These show pressure 

 phenomena of all grades, from slight, wavy extinction to complete granula- 

 tion. Many of the large fragmental quartzes are mashed into oval-shaped 

 areas or are broken into numbers of fragments. The large feldspars are 

 broken, and are altering to quartz and secondary clear feldspar Avith a 

 simultaneous production of epidote and mica. In their least altered condi- 

 tion the original feldspars are cloudy, and hence may be readily distin- 

 guished from the limpid secondary grains. 



The small mineral particles of the matrix, including the mica, do not 

 show undulatory extinction like the large fragmental quartzes and feldspars. 

 These micaceous minerals are in automorphic plates, and wrap around the 

 quartz grains, and in some cases likewise project into them. These con- 

 stituents of the matrix are all believed to be secondary minerals derived 

 from the original clayey matrix, and from the alteration of the feldspar 

 frao-ments, with the possible addition of infiltrated material. At places all 



