162 W. O. CROSBY — ARCHEAN-CAMBRIAN CONTACT IN COLORADO 



sometimes cemented to a firm sandstone or quartzite. Higher up, where 

 it must liave ])een deposited in deeper water offshore, it becomes more 

 or less ferruginous and glauconitic and then calcareous; but the sand- 

 stone is throughout surprisingly free from feldspar and mica, the almost 

 complete absence of arkose material being, as previously noted, the 

 strongest point of contrast between the Cambrian and the sandstones of 

 the higher formations. The Cambrian of the Manitou area is not unique 

 in this respect, for its composition here accords perfectly with m}'- obser- 

 vations in the valley of Eagle river and in the Black hills and with the 

 published descriptions of the Cambrian throughout the Rocky mountain 

 region and eastward to the Champlain valley and beyond. Thus Logan, * 

 in describing the Potsdam (Cambrian) of the Saint Lawrence valley and 

 the northern side of the Adirondacks, saj's: 



" Tlie thoroughly rounded form of the grains of sand composing a large portion 

 of the deposit, and the fact that all the material other than quartz has been bruised 

 up and washed out from so much of it, would seem to make it probable that the 

 formation accumulated slowly, and that the Potsdam [Cambrian] coast remained 

 unchanged for a great length of time." 



These two main facts — a plane erosion surface and the generally non- 

 arkose character of the overlying sediment — must, then, be correlated as 

 complementar}^ [)hases of the erosion process and as testifying with equal 

 distinctness to an extremely slow subsidence of the pre-Cambrian land 

 beneath the Cambrian sea. In other words, the transgression of the sea, 

 which, as suggested by Walcott, probably occupied the whole of Cam- 

 brian time, was so slow and gradual that erosion could, at most points, or 

 save where the sea encountered rocks of exceptionally resistant character, 

 like the Baraboo quartzite of Wisconsin, accomplish its most perfect work 

 in all its phases — planation, reduction of the resulting debris to its simplest 

 and final terms (quartz-sand and clay), and the thorough assorting of these 

 products, so that the ciav was deposited only in deep water remote from 

 the shore along which the sand slowly accumulated. 



Merrill t has covered this ground in part by suggesting a dual correla- 

 tion of the arkose character of sediments with climatic conditions and 

 the rate of deposition. According to the first, arkose indicates a pre- 

 dominance of disintegration over decomposition in the decay of rocks, 

 and hence a dry or frost}' climate, and, vice versa, the absence of arkose 

 in sediments is indicative of decomposition as the dominant erosion 

 factor, and hence a warm and humid climate. The alternative correla- 

 tion implies that deposition may follow so closely upon disintegration as 



* Geology of Canada, 186.3, pp. 108, 109. 

 t Bull. Geol. Soe. Am., vol. 7, p. .362. 



