XXXIV OITTLINE OF THIS MONOGUAPH. 



iU-p.essiou,oCL'Uiiie(l by a swamp or by a stream, exists along the coutact between tbe Arcbeau and 

 the Algonkian. Petrographically the Archean consists of (1) granites or granitic gneisses, (2) 

 "■neisses, (3) mica-schists, (4) horubleude-gueisscs, or amphibolites. The granites possess the nsual 

 characters of such rocks. The gnelssoid members of this division are merely crushed granites, 

 and are connected with the massive rocks by indistinguishable gradations. The gneisses are banded 

 laminated rocks, the minerals of which have crystallized in parallel elongated forms. Subsequent 

 to crystallization they have been acted on by great stresses. The mica-schists are even, mediuni- 

 o-rained rocks, with generally well-developed schistosity. The original chai'acter of these schists 

 is wholly indeterminable. Their relationship with the granites and gneisses is perhaps a reason 

 for ren^arding them as derived from originally massive granites by dynamic metamorphism. The 

 hornblende-'>-neiHses, or amphibolites, are black or dark-green rocks, which are universally foliated. 

 They occur in narrow bauds in the granites and gneisses. Their boundaries are sharp and frequently 

 cut the foliation of the amphibolites and of the gneisses. The field relation as well as the composi- 

 tion of the amphibolites leads to the conclusion that they are old dikes of basic rocks which have 

 been metamorphosed and recrystallized. 



Section I\' treats of the Sturgeon quartzite. The Sturgeon formation is the most widespread 

 member of the Algonkian series in the Felch Mountain range. It occurs in two parallel zones of 

 varying width, immediately adjoining the Archean to the north and to the south, except when 

 displaced from this position by faults. It is fairly well exposed. It frequently forms distinct lineal 

 ridn-es, which, with but few exceptions, seldom rise to the mean altitude of the adjoining Archean. 

 Owing to the completeness of recrystallizatiou, the original sedimentary features have almost been 

 obliterated, so that it is difficult to find places suitable for dip observations. Sufficient dips have 

 been found to show that subordinate folds occur within the quartzite. The average thickness is 

 probably not less than 450 feet, and may be more. Petrographically the formation includes massive 

 quartzites and mashed qnartzites or micaceous quartz-schists, in some of which the relations of the 

 quartz present unusual features. 



Section V. The Randville dolomite, consisting of crystalline dolomitic rooks, overlies tbe 

 Sturo-eon quartzite. The Randville dolomite covers a larger share of the surface in the Felch Moun- 

 tain range than any other member of the Algonkian. Natural exposures are fairly numerous and 

 very evenly distributed. Moreover, test pits and diamond-drill borings have shown the presence of 

 the formation in the covered areas. Relatively the dolomite is a weak rock, and occupies relatively 

 low "-round. An average thickness of 700 feet is estimated for the Randville dolomite within the 

 Felch Jlountain range. Petrographically the formation consists of a rather coarse-grained, thoroughly 

 crvstalline dolomite, with more or less abundant crystals of tremolite and a number of other minerals 

 of minor importance. 



Section VI. The Mansfield schist is only exposed in certain test pits. Its presence has also 

 been determined by diamond-drill borings. The thickness is so small— not more than 200 feet— that, 

 though it weathers readily, it produces no noticeable effects on the general topoarraphy. Petro- 

 graphically it consists of fine-grained mica-muscovite or mica-biotite-schists, probably derived from 

 the metamorphism of a clast'c. It shows nothing of especial interest. 



Section VII. The Groveland formation is magnetic and has been traced by means of compass and 

 dip needle. Excelleut natural as well as numerous artificial exposures render the data concerning the 

 distribution of the formation very satisfactory. The most prominent hills in the Algonkian belt owe 

 their relief to the fact that they are underlain by the Groveland formation. Petrographically we may 

 recognize two main kinds of rock. The usual kind consists of quartz and the anhydrous oxides of 

 iron, while the other and much rarer consists essentially of an iron amphibole with quartz and the 

 iron oxides as associates. Botli of these kinds are clearly of detrital origin. The conclusion is 

 reached, based on certain microscopical structures, that iron and silica were originally present 

 largely in the form of glauconite. 



Section VIII. Mica-schist and ferruginous quartzites of the Upper Huronian series occur in 

 the eastern part of the Felch Mountain range. The rocks constituting the series are soft iron- 



