208 



A. W. G. WILSOX TRAP SHEETS OF LAKE XIPIGOX BASIX 



Lawson cites the local unconformity between the diabase and the sedi- 

 ments as an example of the intrusion of a laccolitic mass of the diabase 

 across the beds. "We have, however, no information as to the actual char- 

 acter of the upper portion of the capping sheet, and in the absence of this 

 positive information the writer would interpret the section differently. 



At first sight the apparent downward dipping of the edges of the beds 

 at the contact with the diabase suggests monoclinal faulting. Directly 

 opposite, across the river, similar sediments also capped unconformably 

 by diabase, form a similar bluff, standing at almost precisely the same 

 level, the distance between the two bluffs being less than a mile. If there 

 were monoclinal faulting, accompanied by the intrusion of the diabase 

 along a plane in the fault zone, not only would the occurrence of escarp- 

 ments of nearly equal height on opposite sides of the fault plane be im- 

 probable, but the lifted block would be the block above the trap sheet, 



Figure 3. — Section at Red Rock at the Mouth of the Xipigon River 



Intersection of Keweenawan strata by diabase, showing a small mass of supposed old soil 

 breccia at the base of a pre-diabase cliff. One hundred and forty (140) feet of sedi- 

 ment are capped by 125 feet of diabase. 



not the one below, and in this case both blocks are below trap sheets. 

 Again, if the molten diabase had been forced upward from below along a 

 fracture plane, it seems probable that if it disturbed the edges of the 

 strata through which it passed it would tend to bend them upward, in 

 the direction of flow, rather than downward. Where the diabase was in 

 a very fluid condition, as presumably it was in this case from the nature 

 of the crystalline structure of solidified magma, where fracturing was 

 produced along the line of the intrusion, small fragments would almost 

 certainly have been washed out into the diabase, and it would have insin- 

 uated itself between other fragments nearer the parent beds, but in no 

 case were phenomena of this character noted. The fragments are fairly 

 uniform in size, no very large blocks being noted, and are frequently 

 rounded at the corners and edges, the whole mass being recemented with 

 a material that was probably derived from the beds themselves. 



The block of sediments now exposed in section above water level very 

 strikingly resembles such a ridge as could be duplicated many times 

 among similar sediments, a flat-topped ridge fronted by a nearly bare 



