DISCUSSION OF EVIDENCE 213 



valleys to be high ridges of diabase rising above the adjacent depression 

 are merely the edges of a dissected sheet of diabase, the present remnants 

 now lying on the sides of old valleys cut in Archean rocks. Since the 

 diabase is harder and more compact near contacts with other rocks, a 

 reasonable explanation of the occurrence of this type of ridge is that the 

 remnant of diabase is preserved where it lies largely because of its rela- 

 tion to the underlying rock and because of the texture thereby developed 

 in this part of the sheet in the process of cooling. The high cliffs at 

 Tchiatang bluff, where there is a relief of over 700 feet and diabase for 

 at least 650 feet of this height, may be a case in point. The whole core 

 of the bluff may possibly be Archean and the diabase only occur as a sort 

 of plaster over its face. 



The third type of unconformable contact has been illustrated by the 

 description of the vicinity of Island portage, on the Mpigon river. 

 Other examples cited are Tchiatang bluff and Humboldt bay, the latter 

 being mentioned in connection with the Ombabika Narrows area. About 

 ten contacts of this type have been examined by the writer, well distrib- 

 uted over different parts of the area, and it has been definitely established 

 that the unconformity they indicate is not confined to any specific 

 locality. 



Discussion. — As has already been pointed out, the first type of uncon- 

 formity has been explained either by the theory of a flow over an eroded 

 surface or by assuming that the diabase was intruded across the beds. In 

 the latter event it appears to the writer that certain evidence of this in- 

 trusion would be found at the contacts in the relationships which would 

 then exist between the intruding rock and the edges of the fractured 

 strata. This direct evidence is lacking, and, so far as the first type of 

 unconformity is concerned, conclusive evidence has not been cited. 



The second and the third types of unconformity have not, so far as 

 the writer is aware, been previously discussed in print. There are but 

 two possible ways in which they can have been brought about. The 

 diabase must have flowed over an eroded surface or it must have been 

 forced in to its present position along the surface of contact between an 

 overlying cover and the rocks with which it is in contact. 



Naturally the surface of contact between the sedimentary rocks and the 

 underlying Archean would have been the locus of a number of lines of 

 least resistance (or a plane of least resistance). Had the Archean sur- 

 face been only moderately even, it is possible that it would have lain in 

 the plane of least resistance; but the surface is an undulating one, with 

 a moderate relief of at least 300 feet and in places more than this. In 

 detail some parts of the surface are extremely intricate, yet we find 



XVIII — Bull. Geol. Soc. Am., Vol. 20, 1908 



