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



oxide and clay, by which, as Russell * has shown, each grain of quartz 

 in residuary earths, and to some extent in the regolith as a whole, is 

 naturally invested. It is, of course, well known that this whitening of 

 sands b}'' the removal of the ferric oxide which coats and stains the 

 grains may be accomplished through the agenc}^ of decaying organic 

 matter, as may often be observed under vegetable mold and peaty de- 

 posits, the iron being reduced to soluble ferrous forms. But in the Man- 

 itou area, at least, it is only in that part of the Cambrian sandstone which 

 is most unquestionably a beach deposit, and where the conditions dur- 

 ing deposition must have been exceptionally favorable to the peroxida- 

 tion and chemical fixation of iron, that the decolorization has occurred ; 

 and here organic matter or any indication of its former presence and 

 iron oxide are equally conspicuous l)y their absence. The only satis- 

 factory conclusion, therefore, is that, as suggested by Russell, the sands 

 were bleached by mechanical attrition. 



In deeper and more quiet water offshore, where the overlying reddish 

 sandstone must have been deposited, enough red clay would also natur- 

 ally have been deposited to fill the interstices between the grains and 

 give the sandstone a ruddy tint in spite of the fact that the quartz grains, 

 in passing through the mill of the ocean beach, have been deprived of 

 their ferruginous cuticles. The calcareous and glauconitic constituents 

 of the red sandstone testify to the comparative tranquillity of the off- 

 shore conditions. The constant and well nigh interminable shifting of 

 the sand on the beach causes any clay that may temporarily become 

 entrapped to be washed out, while offshore the sand is slowly deposited 

 grain by grain, with, as a rule, little subsequent disturbance, and the 

 dead water due to the interstices and to surface friction makes the per- 

 manent entrapment of a certain proportion of clay inevitable. Hence 

 the conclusions appear to be warranted that the ideal beach deposits, 

 especially with a slow rate of subsidence, are white (free from clay and 

 iron oxide), and that red sandstones, not composed of red feldspar, 

 etcetera, may be due to the rapid deposition with little surf action of the 

 clay coated or ferruginous quartz grains of the regolith, as suggested by 

 Russell, or to the admixture of clay with sand which has been bleached 

 by the surf during its slow deposition under offshore conditions. 



*Bull. 52, U. S. Geol. Survey, p. 44. 



