82 J. M. Clements — Study of Contact Metamorphism. 



as lenses agreeing in strike with the sedimentaries, we find 

 slaty siderite, ferruginous chert and iron ore. The origin of 

 the siderite and chert cannot be stated with certainty. The 

 ore was derived from them by processes of metasomatism and 

 concentration. All of these rocks, wherever exposed, are 

 found to strike north and south, with at times slight variations 

 from this direction, probably due to minor crumpling of the 

 formation. The dip is high to the west at about 80°. 



Overlying the Mansfield slates is the volcanic Hemlock 

 formation. 



Immediately east of the slates, that is, stratigraphically 

 under them, there occur continuous masses of coarse dolerite 

 (diabase), which cut off the slates in their strike, both to the 

 north and to the south. ~No well-characterized dikes of dolerite 

 cutting the Mansfield formation have thus far been found, 

 either by surface studies or by underground exploration. In no 

 instance, moreover, has a direct contact between the dolerites and 

 the slate formation been observed, owing to the contacts being 

 covered by disintegration products, although in many instances 

 these rocks are separated from each other by spaces only a 

 few feet in width. That the relations between the two, how- 

 ever, are those of igneous intrusion, is shown by the fact that 

 the dolerites cut across the strike of the slates. Furthermore, 

 if we make a traverse from the dolerites westward into the 

 slates we notice the following. On the westward flank of the 

 dolerite ridge we find associated with the dolerites masses of 

 hard, peculiar, hornstone-like rocks, which have a well-banded 

 character and in places are of large size and very numerous. 

 Leaving this zone we reach the valley proper, in which lie the 

 normal sedimentaries of the Mansfield formation. It thus 

 appears that we have an intermediate zone between the doler- 

 ites and the slates. The banded rocks forming, with the asso- 

 ciated dolerites, this intermediate zone, are believed to be 

 sedimentary rocks which have been metamorphosed by the 

 dolerites. 



The exact original characters of these sedimentary rocks 

 cannot be stated with positiveness. They are believed to have 

 been slates having essentially the same general physical charac- 

 ters and mineral and chemical composition as the slates which 

 now lie next to the contact zoue and extend away from it for 

 several hundred feet across the strike, without showing any 

 noticeable changes in character. Although original chemical 

 variations exist in these slates, as is shown by the banding, and 

 did exist in the slates from which the metamorphosed products 

 to be described were derived, as is shown by the banding in 

 them, — nevertheless, such initial variations in these rocks is of a 

 quantitative rather than qualitative character, and it is not 



