438 THE CRYSTAL PALLS lEON-BEAKING DISTRICT. 



DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. 



The Mansfield rocks of the Michig-amme Mountain area consist of 

 phylUtes or mica-slates of various colors. They are found in the sei'ies of 

 east-west synclines, which have already been described in connection with 

 the Randville formation. The best exposures occur in sec. 32, T. 44 N., 

 R 31 W., between the center and the west quarter post, and still farther 

 north along the south bank of the Michig'amme in the northwest quarter of 

 the section. They are also found round the western edg-e of the Michi- 

 gamme Mountain syncline in sec. 33, T. 44 N., R. 31 W., and in sec. 5, 

 T. 43 N., R. 31 W., but here the exposures are mainly in test pits. Test 

 pits have likewise penetrated them in sec. 10, T. 43 N., R. 31, where they 

 succeed the dolomites as the surface rock over the general arch. Their 

 extent in the covered portions of this area is probably considerable, but 

 the structure is so complex and the outcrops so few as to forbid any but the 

 most approximate outlining of their general boundaries. 



The geological position of the Mansfield rocks is free from doubt. In 

 the principal syncline of section 32 they are seen to overlie the dolomites 

 and to pass downward into them by a relatively slow gradation, while on 

 the borders of the Michigamme Mountain syncline they are proved to 

 underlie the Groveland formation. The passage to the higher formation 

 likewise is graded, though more rapidly, and is marked in certain bands by 

 an increase in clastic quartz grains and by changes in the character of the 

 matrix in which these are set. 



The portions of the surface underlain by the Mansfield formation are 

 without special features, and are indistinguishable topographically in the 

 gently rolling plain, the greater portion of which is formed in the dolomites. 

 In section 32 the outcrops are miniature ridges elongated with the strike, 

 the height of which, however, is less than the contour interval of the map. 



FOLDING AND THICKNESS. 



The folding' of the Mansfield rocks, so far as it can be determined 

 in this area, has already been described in the account of the preceding 

 formation, which they overlie. The rocks are knovvn only in the sec- 

 ondary synclines which lie transverse to the general direction of the 

 main axis south of the Michigamme River. In the southern of these syn- 

 clines, in sec. 32, T. 44 N., R. 31 W., between the limestone rims on the 



