114 BULLETIN OF THE ESSEX INSTITUTE. 



rocks these bodies are as typical dikes as any of igneous 

 origin. " 



2. The rock of the dikes is a fine and even-grained 

 aggregate of sand grains varying in degree of indura- 

 tion from a normal sandstone to a dense hard quartzite, 

 but throughout of a remarkably massive and uniform 

 character. The induration is mainly due to limonite ; in 

 the quartzitic portions, however, there appears to be 

 some secondary silica, although a distinct enlargement of 

 the quartz grains is rare. 



During the past summer of 1896 I was able to devote 

 several weeks to the investigation of the sandstone dikes 

 and the great displacement to which I have found them to 

 be genetically related. To the dikes described by Cross 

 I gave only sufficient attention to become familiar with 

 their characteristics ; and then endeavored to trace the 

 series southeastward through Ute Pass to Manitou and 

 beyond. 



The sedimentary formations of the Manitou area em- 

 brace, from below upward, as described by Hayden, 

 Cross, and others : 



1. A basal sandstone which is usually forty to fifty 

 feet thick, white or gray for the lower ten to fifteen feet 

 and dull red or brown above, only rarely of arkose 

 character, l)ut frequently more or less glauconitic. 

 2. This sandstone, which may be referred provisionally 

 to the Potsdam, becomes calcareous upward, passing into 

 red, cherty limestones, and these into a massive gray 

 limestone having a thickness of several hundred feet. 

 The limestones are throughout more or less magnesian 

 and contain recognizable traces of a Lower Silurian 

 (Ordovician) fauna. 3. This great Manitou limestone 

 series is overlain without apparent unconformity by the 

 Fountain (Carboniferous) beds, one thousand to possibly 



