January 17, 1902.] 



SCIENCE. 



87 



fossils which have been determined by Mr. 

 Walcott as identical with those discovered 

 by him in the pre-Cambrian Belt forma- 

 tion. Thus the discordance corresponds to 

 an hiatus of all of the Paleozoic and part 

 of the Mesozoic. The plane of overthrust 

 dips gently to the southwest, and is ex- 

 posed at right angles to its strike through- 

 out a section seven miles in length, which 

 is equivalent to a displacement of that 

 amount. There are interesting details of 

 structure in the overthrust and under- 

 thrust masses. 



On the western side of the range parallel 

 to the valley of the North Fork of the Flat 

 Head the ancient limestones and quartzites 

 present a bold face, and the stratigraphic 

 relations of rocks found west of the Flat 

 Head valley indicate that this face is a 

 deeply eroded fault scarp of the normal 

 type. The valley of the North Fork of the 

 Flat Head contains lake beds, which are 

 by analogy with similar formations in Mon- 

 tana tentatively referred to the Miocene or 

 Pliocene. 



From these data it is inferred that the 

 structural history of the range comprises: 



First. Deposition of Cretaceous sedi- 

 ments of very considerable thickness ad- 

 jacent to a shore not far from the present 

 site of the range and upon a land whose 

 surface consisted of the pre-Cambrian 

 limestones and quartzites. 



Second. That in some post-Cretaceous 

 epoch compressive strains resulted in a 

 fold overturned toward the northeast, and 

 ultimately in the development of a corre- 

 sponding overthrust fault. 



Third. That at some later date, probably 

 Miocene, normal faulting resulted in rel- 

 ative uplift of the mass of the front range 

 and downthrow of the mass of the Flat 

 Head valley. 



The next paper was a continuation and 

 illustration of the preceding one, numerous 

 lantern views being shown : 



Physiography of the Northern Rocky 

 Mountains: Bailey Willis, Washing- 

 ton, D. C. 



Professor Coleman discussed the physi- 

 ography and origin of the structure of the 

 region to the north of that described by 

 Bailey Willis. Professor Davis discussed 

 the structure and physiography of the 

 region described. Mr. Walcott compared 

 the section of pre-Cambrian rocks of the 

 Belt Mountain terrane with that given by 

 Willis in the Northern Rockies, and con- 

 sidered the probability that the entire 

 series involved in the front range is 

 Algonkian. 



The Walls of the Colorado Canyon: W. M. 



Davis, Cambridge, Mass. 



The general profile of the canyon walls 

 depends on rock structure, and not on a 

 pause in the elevation of the plateaus. The 

 variation of profile from the narrow canyon 

 in the Uinkaret plateau to the wide 

 canyon in the eastern Kaibab is due to 

 variation in the character of the strata. 

 The pattern of spurs and recesses varies 

 with the stage of dissection. The pattern 

 commonly seen in the Red-wall cliffs is re- 

 peated in the Tonto cliffs where the latter 

 are much worn. The pattern usually seen 

 in the Tonto is repeated in the Red-wall 

 where it is less worn. Brief mention was 

 made of details connected with the uncon- 

 formities seen in the canyon walls. 



The paper was illustrated by lantern 

 views, and was briefly discussed by Mr. 

 Walcott and others. 



The Society then adjourned for the 

 noon recess. 



The papers of the afternoon session 

 were: 



Bach Basins at the Helen Mine, Michipi- 

 coton : A. P. Coleman, Toronto, Canada. 

 Two small lakes or ponds, each a quar- 

 ter of a mile long and two-thirds as wide, 

 just west of the Helen iron mine near 



