36 THE GEOLOGY OP MINNESOTA. 



[The Kekequabic granite. 



spar. It is seen in the tuff in No. 1093, and in the granite in No. 1088. In the por- 

 phyry this fine matrix appears to be a micro-granulitized feldspathic debris, for 

 numerous feldspars can be seen partially changed to such a micro-granulitized condi- 

 tion (No. 1093). Rock No. 1051 shows a link between the schist and the granite. It 

 overlies green schist, and unites Nos. 1044, 1045 and 1046 to Nos. 1047-1051. 



Still, under the hypothesis adopted it is not to be expected that very many 

 actual transitions in situ from a plainly clastic rock to a plainly igneous one could 

 be observed in the field. The moment a clastic rock becomes sufficiently plastic to 

 move, under the pressure exerted upon it, it is likely to be dislodged and to enter 

 any fissures that may exist in the firmer rocks surrounding it; and that gives it 

 " igneous contacts " on the elastics. With these clastic rocks it would hence present 

 not only contrasting structures, but also more or less petrographic differences. The 

 porphyry here considered penetrates, in a few instances, the black slates associated 

 with the green schists and with the gray wackes, in the form of dikes, the slates being 

 firm and refractory, affording most evident illustrations of intrusive action. In the 

 area of the softer green schists, however, which are not brittle, but rather flexible, 

 the same intrusive appears in the form of more or less round protuberances and blebs. 

 It is, however, with the conglomeratic condition of the sedimentaries that the 

 most evident transitions occur to the granite. These are most evident in the field, 

 and with the microscope the finer elements are seen to be simply compacted together, 

 with but slight interstitial material. The crystals all being, as supposed, of the nature 

 of volcanic ejecta, arranged somewhat by water, the elements of the rock embrace 

 these with small amounts of erosion products, the latter increasing in amount with 

 distance from the supposed volcanic source. One of the most evident and instructive 

 instances of a granitized conglomerate is that seen along the south shore of the 

 lake in sec. 31 T. 64-6. The fragmental character is most evident, and many of the 

 pebbles are rounded. There is no short transition, but the whole rock over a wide 

 belt acquires the granitic character. The appearance of the rock along this belt has 

 been described as follows* by Dr. Grant: 



On the northwest end of the little point in the S. W. % of N. W. % sec. 31, T. 65-6 (south side of the lake), 

 there is a dark, medium grained diabase. And on the northeast corner of this point is a low outcrop of a tine 

 grained, gray, apparently holocrystalline rock: the ground-mass is grayish and in it are small, black needles, 

 probably of hornblende, and a few scattered, rather irregularly outlined, feldspar individuals. There are also a 

 few rounded pebbles, up to those two inches in diameter, scattered through the rock. The specimens collected 

 (No. 593G) show some of the pebble forms. Some of these pebbles are seen to be sub-angular, but most of them 

 are rounded. They seem to be scattered irregularly through the rock and lie in no definite planes or layers ; 

 there is nothing in the rock to show any sedimentary lamination or bedding ; it appears perfectly massive. This 

 rock is seen in several outcrops in the N. E. % of S. W. % sec. 31, T. 65-6, and the shore is here usually lined with 

 fragments of it. In the eastern part of this one-sixteenth section is quite an extensive exposure a short distance 

 back from the shore. Here the pebbles, which have been steadily increasing in abundance eastward from the 

 first mentioned outcrop, are very numerous. It would be almost impossible to find any surface a foot square in 

 the rock at this place which would not contain one or more pebbles, and many areas of this size would include as 



'Twentieth Annual Repurt, p. 78. 



