RELATION OF FORM OF CONTACTS TO SEDIMENTS 163 



to forestall decomposition. This must mean, however, that, as would 

 doubtless be generally conceded, subaqueous are less favorable than sub- 

 aerial conditions to the chemical decomposition of mechanical detritus, 

 and suggests a sharper distinction than is commonly made in this con- 

 nection between subaerial and marine erosion. The former yields, ac- 

 cording to climatic conditions, arkose or non-arkose detritus, and the 

 arkose detritus yields, according to the rate of deposition, arkose or non- 

 arkose sediments. The essentially unaltered condition of arkose sedi- 

 ments in all geological formations proves that the process of kaolinization 

 is practically at a standstill on the marginal sea-floor. Daubree and 

 others have shown, on the other hand, that the fine trituration of sili- 

 cates, even in pure, cold water, is inevitabl}^ attended by liberation of 

 the alkalies and alkaline earths and the formation of kaolin as the final 

 residue. To summarize: arkose in sediments implies rapid deposition 

 of mechanical detritus — the product of subaerial or marine erosion— and 

 the absence of arkose is consistent with either slow or rapid deposition 

 of the subaerial chemical detritus, or the slow deposition of mechanical 

 detritus from any source ; but it must alwaj^s mean slow deposition for 

 sediment in close proximity to an erosion unconformity, as is the basal 

 member of the Cambrian sandstone, and then the unconformity should, 

 theoretically, be of an approximately plane or baseleveled and feature- 

 less character. 



Not only is the basal member of the Cambrian sandstone in the Man- 

 itou area almost absolutely free from arkose material, but it is also, as a 

 rule, remarkably well assorted— mechanically pure as well as chemically 

 pure — and evenness of grain does not here mean exceptional fineness of 

 grain, although the sandstone usuall}'- becomes somewhat finer upward 

 from the base. This even texture or mechanical homogeneity, expressed 

 also in the practical absence of conglomerate and grit layers, is addi- 

 tional proof that the sedimentary process was deliberate and thorough, 

 and it is probably a safe correlation, well sustained and illustrated by 

 a comparison of the Cambrian and Fountain beds of the Manitou em- 

 bayment, that arkose sediments are in general ill assorted, and vice versa. 

 It may be noted, however, that to some extent a limitation upon the 

 textural homogeneity of sandstones, through any considerable thickness, 

 is imposed by the condition that below a certain size effective abrasion 

 ceases, neither the size nor the angularity of the grains suffering appre- 

 ciable diminution with continued trituration. 



The gray to white color of the basal sandstone is believed to have a 

 like significance, indicating not only the complete elimination of the 

 feldspar and mica of the granitic debris, but also the removal by gentle 

 and loug continued attrition of the more or less distinct pellicle of iron 



