5l6 SHAW: SEDIMENTATION 



round and frost the grains + a transgressing sea to redistribute 

 the sand without introducing any more mud than the formation 

 contains, and so on; but at the end we would still have a very- 

 poor idea of the conditions of deposition. Indeed if each known 

 element in the equation were fully understood and were repro- 

 duced we probably should not get another St. Peter sandstone, 

 and possibly the result would only remotely resemble it. 



The Devonian black shale, the great coal beds with their 

 persistent thin partings, the wide-spread sandstones, such as 

 the Dakota and Berea, the oolites, the so-called "Lafayette 

 formation," and a great many others are still more or less mat- 

 ters of mystery. We are inclined to make remarks of satisfac- 

 tion such as that we were formerly misguided regarding a cer- 

 tain formation but that we know better now, — we understand 

 the conditions of its deposition; we used to think, for example, 

 that coal is a product of tropical climate; now we know that it 

 is formed under temperate or cool temperate conditions. But 

 is there not still a great gulf between the relatively small and 

 rarely smothered peat bogs upon which our assumed under- 

 standing of coal rests and those great expanses of peat swamp 

 which developed over a sinking and frequently flooded land? 



It seems to me that after all, we know for most strata only 

 the main features of their conditions of deposition. We do not 

 know whether the surficial sand and gravel deposits of the 

 Atlantic Coastal Plain are mainly marine or mainly fluviatile, and 

 this, notwithstanding the fact that all are geologically recent 

 and some very recent formations. Various criteria become ob- 

 scure with age and in particular the related physiographic fea- 

 tures are gradually wiped out. Yet here in spite of youth and 

 more or less perfect preservation of several kinds of physio- 

 graphic features we are still groping in the dark, and are for- 

 tunate if we are able to avoid the pitfall of jumping at con- 

 clusions. 



Vaughan^ says: "It is generally agreed that the soundest basis 

 for inferring the condition under which past sediments were 

 formed and deposited is to be obtained through a study of pres- 



^ Unpublished memorandum to National Research Council. 



