64 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



igneous rocks and .slates are in contact or in close association, the latter 

 have been metamorphosed, and adinoles, spilosites, and desmosites have been 

 tbrnied which are similar to those described as occnmng- in other areas 

 along the contact zone of basic iutrusives. Although no single instance of 

 a dike penetrating the slates has been found, it can hardly be doubted from 

 the relations which have been oiitlined that the slates are older than the 

 inti'usive dolerites. 



RELATIONS TO VOLCANICS. 



The sedimeutaries are overlain by volcanics, both lava flows and tufa- 

 ceous deposits. In these tuffs, at the northeast corner of sec. 7, T. 43 N., 

 R. 31 W., angular black-slate fragments have been found similar in every 

 respect to the slates of the Mansfield belt. From this it is clear that at 

 least some of the volcanics are younger than part of the slate formation. 

 In section 29 similar relations obtain, the only difference being that the 

 masses of slate and graywacke are inclosed in rather larger fragments in a 

 volcanic conglomerate, and still retain very closely their normal strike. In 

 the conglomerates near the Mansfield mine are found chert fragments and 

 in some places fragments of iron oxide. These latter were evidently not 

 included as oxide, but as fragments of cherty carbonate. Like the great 

 mass forming the ore body, the fragments have since their deposition been 

 altered, forming iron-oxide bodies of small size. Further discussion of the 

 relations between the volcanics and slates will be found under the heading 

 " Hemlock foi'mation." 



STRUCTURE OF THE MANSFIELD AREA. 



It has already been seen that the Mansfield rocks strike north and 

 south and have a high westerly dip. The two possible explanations of this 

 structure which are compatible with the facts in other portions of the area 

 are (1) that they form a westward dipping monocline, and (2) that they are 

 the western limb of an anticline. 



THICKNESS. 



As the sedimeutaries forming the Mansfield belt now dip west at a very 

 high angle, and as there is no evidence of duplication of strata due to fold- 

 ing, I feel comparatively safe in giving an estimate of their thickness. The 

 belt is widest at the north end, and there has a breadth of about 1,950 feet. 



