PBTEOGKAPHICAL CHAEACTEES OF MANSFIELD SLATE. 63 



RELATIONS OF SIDERITE-SLATE, FERRUGINOUS CHERT, AND ORE BODIES TO 



CLAY SLATES. 



Owing' to the scarcity of the outcrops of the sedimentaries in the Mans- 

 field Valley, it is practicall}" impossible to decipher the relations of the 

 individual beds. Neither the study of the surface exposures nor the expo- 

 sures in the mine workings have given definite results. That the beds repre- 

 sent interbedded strata is well understood, but the sequence of the strata is 

 indeterminable. It is of especial interest to determine^ so far as possible, 

 the relations of the ferruginous rocks, in order that the possible iron-ore 

 deposits associated with them may be found. A cross section tlu-ough the 

 Mansfield mine from east to west shows the following relations: The foot- 

 wall of black hematitic slate is overlain by 25 to 30 feet of ferruginous 

 chei't and iron ore. This stratum is succeeded by "red slate," so called by 

 the miners, which is probably weathered greenstone impregnated with iron. 

 This is followed by a conglomerate, and this by amygdaloidal greenstone, 

 of the overlying volcanic formation. The ore body extends north and 

 south, agreeing thus with the strike of the slates. All drifts end on the 

 north in mixed ore, and on the south in mixed ore, with "quartz-rock" and 

 "lime-rock" of the miners in some places. From these facts we may justly 

 conclude that the ore-bearing ferruginous cherts exist in beds in the slates 

 or as lenticular masses which agree in dip and strike with the surrounding 

 slates. This conclusion is confirmed by test pits along the strike of the 

 exposed beds, which have disclosed similar ferruginous cherts at various 

 places for a distance of half a mile to the north. 



RELATIONS OF MANSFIELD SLATE TO ADJACENT FORMATIONS. 



RELATIONS TO INTRUSIVES. 



The Mansfield slates are surrounded on three sides — east, north, and 

 south — by coarse-grained basic eruptive rocks. The fact that they are so 

 surrounded by these rocks, which cut them off' in the direction of their 

 strike, points to the later origin of these eruptives. Moreover, the quartzitic 

 character of some of the sedimentaries shows that they could not have been 

 derived from the eruptives which stratigraphically underlie them, for in 

 these no quartz is found. The quartzitic character would thus seem also 

 to indicate that the slates are older than the intrusives. Wherever the 



