THE UPPER SLATE MEMBER. 838 



having feldspar predominant, little or no quartz being present (PI. xxxiii, 

 Fig. 1). The fehlspars coniniouly comprise orthoclase, microcline, and 

 plagioclase. Usually some of the feldspars are altered to a greater or less 

 extent to chlorite, sericite, muscovite, biotite, and kaolin. In the inter- 

 stices there may be a small quantity of finely crystalline fjuartz. In many 

 cases, however, the graywackes and graywacke-slates are not so simple in 

 composition. Mingled with the larger particles of fragmental quartz and 

 feldspar are finer particles of the same sort, with other minerals (PI. 

 xxxii, Fig. 4). When this finer silt is preponderant the graywackes and 

 graywacke-slates grade into the clay-slates or phyllites. Naturally, in a 

 belt in which the above described simple graywackes and clay-slates both 

 occur there would be found gradations between them. Frequently in the 

 same specimens we find mingled coarse and fine material. As the quantity 

 of finer material increases it becomes increasing'lv difficult to trace the 

 exact changes which have taken place in the minerals. Unless all of the 

 material is excessively fine, i. e., unless the rock passes into a clay-slate of 

 the finest possible sort, the processes of change subsequently described are 

 seen to have taken place with the larger particles of (juartz and felds]iar in 

 these fine grained slates. The graywackes, besides showing great variation 

 in appearance, due to the mingling of coarser and finer material, occa- 

 sionally contain so nuicli ferrite — brown iron oxide — as to make this sub- 

 stance a chief constituent. Pyrite, some carbonate (calcite, dolomite, or 

 siderite), and rarely carbonaceous material are also quite often present as 

 more or less plentiful accessories. 



(1) Quartzose graywacke (PI. xxxii). — In the freshest of the quartzose 

 graywackes many of the particles of feldspar are as unaltered as in 

 ordinary granite, but generally they have decomposed to a greater or less 

 extent. This decomposition has ordinarily taken place to a greater degree 

 near the exteriors of the feldspar particles than in their centers. In the 

 particles in which tlie decomposition has been somewhat extended the 

 alteration has affected the regularity of the original fragmental oval out- 

 lines. Upon the other hand, even when alteration lias pi-ogressed quite to 

 the centers of the feldspars, the original outlines may at times be (piite 

 sharp. The minerals which have resulted from the pai'tial alteration, and 



