April 16, 1897.] 



SCIENCE. 



607 



tion. Second, the cracks post-date the depo- 

 sition of the sand, but antedate its lithifi- 

 cation to form a firm sandstone and the un- 

 consolidated sand subsided and flowed down 

 into and filled the fissures. 



By this process of elimination we are 

 forced to the consideration of the view that 

 the fissures were formed after the granite 

 had been covered by the sedimentary de- 

 posits and before their complete consolida- 

 tion, the unconsolidated portions naturally 

 contributing to the filling of the fissures 

 and the formation of the dikes. There are 

 two questions especially which the accept- 

 ance of this explanation would require to 

 be answered in the af&rmative. First, are 

 there, among the sediments of the Manitou 

 and Manitou Park basins, any that, aside 

 from structural features like stratification, 

 which would, of course, be obliterated dur- 

 ing the filling of the fissures, present a 

 reasonably close agreement in character 

 (composition and texture) with the sand- 

 stone of the dikes? Second, may we 

 reasonably assume that these sediments 

 were, in part at least, unconsolidated or 

 Imperfectly consolidated at the time when 

 the fissures were formed ? The only sand- 

 stone formations that need be considered in 

 this connection are the Potsdam, Carbon 

 iferous, Triassic and Dakota. Of these 

 four sandstone horizons the last three bear 

 no special resemblance to the material of 

 the sandstone dikes. On the other hand, I 

 became convinced before the field work was 

 finished that the Potsdam beds and the 

 dikes are lithologically identical. The dike 

 rock is absolutely indifferent to the changes 

 in the character of the neighboring forma- 

 tions, showing no appreciable variation, as 

 in succession, from Manitou southeast to 

 Cheyenne Cafion, the Potsdam, Silurian, 

 Carboniferous, Triassic and Dakota beds 

 abut against or border the great fault. 



The close association of the dikes, 

 throughout the entire belt, with the great 



displacement, and their unvarying litho- 

 logical similarity to the Potsdam sandstone, 

 have suggested to me that the dikes prob- 

 ably date from the formation of the Ute 

 fault ; that the fault probably dates from 

 the time when the Potsdam beds, which are 

 still, at the base, in part of a more or less 

 friable character, were imperfectly consoli- 

 dated and covered the entire region ; that 

 the fault, as is likely to be the case with a 

 great displacement, was not simple, but 

 that a moderate breadth of the granite and 

 overlying formations was traversed by a 

 series of parallel fissures ; and that the 

 dikes resulted from the sinking of the Pots- 

 dam sandstone and sand into the fault fis- 

 sures. Such local subsidences of the friable 

 sandstone would naturally be attended by 

 a more or less complete obliteration of the 

 bedding. 



No single feature of the dikes is more 

 significant than the great breadth of indi- 

 vidual examples. Although presenting, 

 apparently, an insuperable obstacle to all 

 the other suggested explanations of the 

 sandstone dikes, it offers no difficulty what- 

 ever to the theory proposed here, for we 

 have only to make the extremely probable 

 supposition that sheets of granite of vary- 

 ing width and bordered by complemen- 

 tary faults have settled down relatively 

 to the bordering masses, bearing with them 

 their loads of Potsdam sediment. 



W. 0. Crosby. 



EXHIBITION m SCIENCE BY THE NEW YORK 

 ACADEMY OF SCIENCES. 

 The Fourth Annual Exhibition of Eecent 

 Progress in Science, given by the New York 

 Academy of Sciences, was held at the Amer- 

 ican Museum of Natural History on April 

 5th and 6th, and was in every way the 

 most successful in the history of the Society. 

 The exhibit occupied the floor space of the 

 main hall and bird gallery, and was at- 

 tended by an estimated number of more 



