THE UPPEE SL-ATE MEMBER. 341 



defined twinning bands and these may be secondary developments. 

 Biotite in well defined folia of varying- size, the larger being about 

 .J nun. in length, is very abundant. Muscovite is nuicli less alnuidant. 

 The folia of mica often cut through the smaller quai-tz-grains and into 

 those of the feldspars, just as in a typical crystalline schist. That much 

 ot the mica is a secondary product can not l)e proved from the sec- 

 tions from the locality of English lake, taken by them.selves. A portion of 

 it is, however, certainly of this nature. Many grains of feldspar are so 

 cut by folia of mica as to make it probable that the latter is a secondary 

 product, while some of the larger particles of feldspar contain throughout 

 their areas folia of mica and grains of quartz which, with the remaining feld- 

 spar, give an appearance of most intricate interlocking, as in no case do 

 the grains of feldspar now show the rounded outlines which thcA' once 

 doubtless had. This mica included in the feld.spar is precisely like that of 

 the remainder of the rock — the word matrix for remainder can hardl)- be 

 used, for in coarseness of grain the parts of the rock which show no trace 

 of fragmental origin are about the same as the quartz and mica included 

 in the feldspars, and which, taken in connection with the rocks previously 

 described, are regarded as pro\ing that this rock, like them, was once 

 altered sediments. Quite numerous crystals of pyrite are seen. 



The rocks (2) to (6), above described, thus constitute a graded series 

 from the fresh feldspathic graywackes to the crystalline mica-schists. The 

 gaps here left in the.se typical descriptions may be filled in by follow- 

 ing the detailed descriptions of the graywackes and graywacke-slates 

 and mica-slates and mica-schists (pages 309-326). Here the wide inter- 

 val between the most })lainly fragmental graywacke.-; and the most crys- 

 talline mica-schists is completely filled by almost imperceptible stages, 

 and the pi'ocesses wliich caused tlie alterations are clearlv indicated; the 

 result helng the production from <i coniplctelif fraffmentul arkosc rock, bji meta- 

 somatic changes only, of a rock ivIiicJi presents ever if appenranrr of cotnpTete 

 original crystallization, and which icoidd be ordinarily classed a.s a genuine crys- 

 talline schist. 



Black mica-slates (PI. xxxiv, Figs. 3 and 4). — The series of alterations 

 above described has also been very important iu the production of various 



