8 PROCEEDINGS OF THE DENVER MEETING 



EFFECT ON THE TRAPS 



One of the most conspicuous features of the faulting is the extremely intense 

 brecciation of the igneous rocks near the contact. This has been so thorough that 

 frequently these rocks are so shattered that a fragment an inch in diameter can not 

 be found which is not crossed by one or more fractures; and this fractured zone 

 extends in some cases more than 400 feet from the junction of the two formations. 

 Generally there has been no appreciable motion of these fragments on one another, 

 but in places the fragments are slickensided, and but rarely has the motion been 

 sufficient to round them. 



EFFECT ON THE SANDSTONE 



The sandstone, except at one locality to be mentioned later, does not exhibit the 

 brecciation common to the traps. The usual altitude of the sandstone near the 

 junction is a bending upward of the layers so that they have a marked northerly 

 dip, but the amount of the dip rapidly diminishes on leaving the contact, and the 

 layers soon assume their normal position of practical horizontality.* Sometimes 

 this simple relation is complicated by minor faulting or folding in the sandstone, 

 the details of which have not been yet worked out. At one locality, however, the 

 sandstone has been more profoundly affected and has been thrown into an anti- 

 clinal fold of considerable dimensions. f The southern limb of this fold, which 

 limb is nearly half a mile across, dips steeply toward the south under the traps- 

 Immediately at the contact there is evidence, as pointed out by Professor C. R. 

 Van Hise, of a small sharp syncline. Here the upper part of the sandstone is as 

 intensely brecciated as are the traps. 



CONTACT PLANE 



In two localities the actual contact plane of the two formations is nearly or quite 

 exposed, and here this plane has a noticeable hade toward the south — that is, 

 toward the upthrow side. This, together with the structures in the sandstone 

 already mentioned, marks the displacement as a reversed or thrust fault, the traps, 

 which are older rocks, being on the south or upthrow side and the sandstone being 

 on the north or downthrow side. 



CONGLOMERATIC BEDS 



By the faulting some conglomeratic beds of the sandstone are brought to view. 

 The pebbles are all well rounded, and are composed mainly of rocks which can be 

 referred to the more firm and silicious parts of the adjacent, but not necessarily 

 immediately adjacent, traps. With these trap pebbles are some of vein quartz, and 

 at one locality pebbles of quartzite, which as far as known cannot be duplicated 

 in this vicinity, are common. Thus the evidence that there was displacement 

 here before the deposition of the sandstone, and that the sandstone was deposited 

 against a fault scarp shore cliff, and that there has been little displacement since, 

 is not clear. The relations here are thus different from those at the junction of 

 the two formations on Keweenaw point. It is only just to state, however, that in 

 Wisconsin all the information to be had from a detailed study of this contact and 

 these pebbles is not at hand. 



* Ibid., p. 19 and pi. 8, section AB. 

 % Ibid., p. 20 and pi. 9, section CD. 



