THE QUARTZ-SLATE IMEMBEH. 151 



whicli the particles are still very fresh and the chloritic slates or the argilla- 

 ceous shales ; the most plentiful products of the decomposition of the 

 feldspars being chlorite and kaolinite. In the decomposition of the feld- 

 spars to chlorite, the particles of the latter mineral are found to form in the 

 lirst place in the neig-Jdjorhood of the edges of the feldspar fragments, the 

 alteration having progressed regularly from tlie outsides of the grains ; in 

 the case of the kaolinitic alteration, the kaolinite particles appear as usual 

 in numerous minute flakes tlu'oughout the entire feldspar grains. 



The large particles ni' white mica which are sujtposed to be of a frag- 

 mental nature are taken to be an altered muscovite. 



The fine interstitial material in these rocks, aside from the minute 

 grains of quartz and feldspar, is a mixture of n minutely divided 

 silica, flakes of kaolinite, chlorite, and a Hue mica, wincli is taken to 

 be sericite and muscovite, in proportions which vary l)etween very wide 

 limits, although in nearly every case all of these minerals appear to he 

 ■ present in the matrix, except perhaps tlie sericite. Tlie siliceous portion of 

 this matrix is at times completely thougli very minutely crystalline, hut in 

 other cases it takes on a chalcedonic or even an amorphous form. While 

 some of the larger particles of the tpiartz in tlie matrix are doulitless of a 

 fra<i'mental nature, much of this interstitial silica is evidentlv an original 

 crystallization. 'I'he kaolinite and chlorite are in minute flakes intimately 

 minjiled with the fine silica ; the brown iron oxide occurs in irregular l)rown 

 patches, being usualh' in a rather minute (piantity, though at times deepl}' 

 staining the entii'e matrix. The sericite particles of the matrix are usually 

 of somewhat larger size tlian tlie particles of tlie other minerals, on which 

 account it is perha))s a (picstion whether this sericite should not rather be 

 regarded as belonging with the larger mica flakes which we liave already 

 mentioned as of a fragmental nature. All of these interstitial minerals are 

 ones which, as is known, i-esult from the alteration of feldspars and micas, 

 and it is supposed that the}' are the result of metasoinatic clianges carried 

 out particularly in the finer detrital material subscfjucntly to the orig-inal 

 ileposition of the rock. 



Microscopical character of the hinfifU (iiid cli/nr/tic (jiKirlz-ahifcs. — (Phases 

 1 and 3, see PI. xix, Fig. 1.) The thin sections of these slates, which are, as a 



