THE KEWEENAWAN. 399 



observed. Beyond Paulson's mine the Upper Huronian sediments begin to 

 widen, rapidly increasing in width as they are followed to the east, as 

 alread}^ described Corresponding with this widening, we find an increas- 

 ing number of sills having in general a trend east and west and lying 

 approximately parallel to each other. During several trips to Gnnflhit 

 Lake and to the country to the south a number of these sills were followed 

 along their strike for short distances and were also crossed at right angles 

 to the strike. Their relations to the sediments were thus clearly seen. 

 No attempt was made to trace out the individual sills. This work has 

 been done' in previous years by Chauvenet" and Merriam,'' of the United 

 States Geological Smwey, and in more recent years by U. S. Grant,'' of the 

 Minnesota Survey. 



The data for the distribution of the siils which are shown on the accom- 

 panying map have been taken chiefl}' fr(_»m the reports of these men. 



JExposures. — Throughout the area iinderlain by the gabbro, as well as 

 the sills, exposures are very numerous and usually of large size, affording 

 excellent opportunities for the study of the characters of these rocks, their 

 variations in grain, and also their relations to the adjacent sediments. 



Toxiography. — The line of contact between the gabbro and the older 

 rocks adjacent to it is fairly well marked by a slight topographic break. The 

 gabbro normally has a steep north face sometimes showing an escarpment of 

 varying height. It is never very high, but is considerably higher than any 

 topographic features in the area north of it for some distance. The contact 

 is frequently marked by a lake or a stream. This difference between the 

 topography of the gabbro area and that to the north exists at the immediate 

 contact, but examining contrasting areas as a whole we find that in general 

 the gabbro area is lower than that underlain by the older formations to the 

 north. Locally the gabbro area has been reduced almost to base level. In 

 fact, this area may be described as veiy nearly a plain, but one with iniiior 

 but pronounced irregularities. The uniformity of the surface is due in great 

 part to the homogeneous character of the gabbro mass, which has caused it 

 to be about equally affected by the various agents which have attacked it. 

 The minor pronounced irregularities are usually found to be due to erosion, 

 which has been controlled very frequently by the joints of the gabbro, and 



«W. M. Chauvenet, U. S. Geol. 8urve}', manuscript notes. 

 b Mon. U. S. Geol. Survey Vol. XIX, 1892, PI. XXXVII. 

 cGeol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, pp. 487-488. 



