TFIE UPPER SLATE MEMBER. 335 



by a secondai-y enlargement of quartz fragments ; the dejiosition or forma- 

 tion in situ of interstitial finely crystalline quartz, accompanied with a 

 micaceous or chloritic alteration of the feldspar — the first two being pro- 

 cesses already fully described by ns.' 



Before following further the series of changes which explains the met- 

 amorphosis of fragmental sediments to mica-slates and mica-schists, it is 

 important to recall the chemical changes" which occur in the alteration of 

 orthoclase, microcline, and oligoclase to chlorite, muscovite, andbiotite. The 

 average content of silica of the following minerals is taken from Dana's 

 System of Mineralogy : Orthoclase and microline, 65 per cent ; oligoclase, 

 62 per cent; muscovite, 45 per cent; biotite, 40 per cent; chlorite, 25 to 

 30 percent. Evidently where the alterations of orthoclase, microcline, and 

 oligoclase to muscovite, biotite, and chlorite have taken place so extensively 

 as in the rocks under discussion, it is not difticult to explain the presence of 

 the silica which lias enlarged the fragments of quartz, replaced those of 

 feldspar, and separated as independent interstitial quartz. One of these 

 alterations is stated by Tschermak' to oecur as follows: "Wenn man die 

 dreifache Formel des Feldsjiathes 3 (Kp'Alp^GSiOJ mit jener des daraus 

 entstandenen Glimmers Kp-Aip3.2SiO,-f 2 (Hp-Alp;2SiOJ vergleicht, so 

 ergibt sich, dass von der urspriinglichen Menge 6*SiO^ m;r 2SiO, in die 

 neue Verbindung iibergehen und 4SiO^ iibrig bleiben." In further speak- 

 ing of the alteration of orthoclase to muscovite, Tschermak also observes: 

 " Der neue entstandene Muscovit ist ofters audi von Biotit (Magnesiaglim- 

 mer) begleitet." 



For the iron of the biotite and chlorite in the rocks under considera- 

 tion, it is not difficult to account. Pyrite, marcasite, and femte are quite 

 widely present in these rocks as accessory constituents. Often the relations 

 of the pyrite or marcasite and biotite (folia of the latter surrounding j)articles 

 of the former) are such as to lead to the supposition that the former min- 



' On Secondary Enlargements of Mineral Fragments in Certain Rocks. R. D. Irving and C. R. 

 Van Hise. Bull. U. S. Geol. Survey, No. 8. 



- 1 insert freely, witlioiit c|iiiit:itions, here .nid in the following pages, such parts of .in article by 

 nie on this subject, already imblished — .Vm. Jour. Sci., 3d series, vol. .\.\.xi, 1886, pp. 453-459 — as can 

 be nsoil. 



' Lehrliueh der MiMer.ilngle, zweite .Vuflage, p. 462. 



