56 THE CRYSTAL FALLS IKON-BE AKING DISTRICT. 



tain/ and tlie slater of the two areas are consid red to be of the name age. 

 Since the slates correspond stratigraphically to the slates of the ]\Iichigamnie 

 ]\Iountain and to those of the Manstield area, they have been connected on 

 the map with the slates of Micliigamme Mountain by a narrow belt included 

 between dotted lines; but this belt is not based on any connecting exposures. 

 These two ledges of slate are taken as the northernmost outcrops of the 

 Mansfield slate formation, although a number of miles north and in direct 

 continuation of them along the strike there was found a single doubtful out- 

 crop of a graj^wacke, showing neither strike nor dip. Whether it represents 

 a shallower water deposit contemporaneous with the slates it is impossible 

 to say. However, on such slight evidence it was not deemed advisable to 

 continue the slate belt to this point. 



PETROGRAPIIICAL CHARACTERS. 



A petrographical description of the Manstield slate belt must neces- 

 sarily be very brief, owing to the small area and to the scarcity of the 

 exposures. 



The rocks of the Mansfield slate belt are graywackes, clay slates, 

 phyllites, siderite-slates, cherts, ferruginous cherts, and iron ores, with the 

 various rocks which have been derived from them by metamorphism. 

 They xarj from coarse-grained rocks to very fine grained slaty ones. The 

 latter predominate, and for that reason this belt is called a "slate" belt. The 

 color of the rocks varies from an olive green and purplish lilack to bright 

 red for those which are very ferruginous and more or less altered. 



The ordinary detrital rocks may be divided into the coarser and the 

 finer kinds. The first are the graywackes, and the second are the ordinary 

 clay slates and ph}'llite. There is, however, a gradation from the one to 

 the other. 



GRAYWACKE. 



The gravwackes consist largely of grains of quartz and feldspar of 

 unquestionabl}' detrital origin. Associated with these is a large amount 

 of mica, chlorite, and actinolite, with invariably more or less rutile. This 

 last is in minute grains as well as in crystals. Many of the crystals show 

 fine knee twins, triplets, and more rarely, heart-shaped twins. Tourmaline 



' See Part II, Chapter IV, Sec. IV, by H. L. Smyth. 



