428 C. SCHUCHERT' DIAGENESIS IN SEDIMENTATION 



which in places is said to be more than 30 feet thick. This is the result 

 of organic decay and is in a state of change. From all these facts we get 

 some insight into the enormous organic chemistry of nature, and thus 

 become aware of the magnitude of its reaction on the inorganic materials 

 of which our sedimentary formations are composed. 



This interaction of organic chemistry on the marine and fresh-water 

 deposits produces in the main the cements that bind the sandstones and 

 mudstones. Through the living organisms are produced the limestones, 

 and through organic decay their change into the magnesian limestones 

 and the dolomites. The carbonaceous materials of the stratiform rocks, 

 including the kerogen out of which petroleum develops, are also of or- 

 ganic origin. The coal beds are but the accumulation^ of land plants more 

 or less decomposed and chemically altered. Denitrifying bacteria are 

 necessary to the formation of some kinds of limestone deposits, while 

 other kinds of bacteria lead to the formation of iron ores and bring about 

 the marcasite and pyrite nodules of black shales. But why do the changes 

 cited, and many other transformations as well, go on in one place and 

 not in another ? Why are some limestones crystalline, and others, which 

 are 97 per cent carbonate of lime, amorphous? Why are some of the 

 dolomites coarsely crystalline or cavernous, and others not so at all? 

 Why are some dolomites rich in the molds of fossils, and yet in others 

 every trace of the organisms is removed by the diagenetic changes? 

 These are but a few of the changes wrought by the decomposition prod- 

 ucts of organisms. 



Even though we can mention many diagenetic changes, we have as yet 

 no clear idea of the processes and hardly any at all as to the depth, 

 clarity or turbidity, temperature, or the agitation or streaming nature of 

 the waters in which these alterations go on. When we know'more of 

 these conditions, to be discerned in the lakes and seas of today or experi- 

 mentally developed in our laboratories, then we will also be able to tell 

 the depth and nature of the waters and the climates under which the 

 deposits were accumulated. Then diagenesis will be another stepping- 

 stone toward a visualizing of the actual physical and organic worlds of 

 the geologic past. 



Finally, let me ask, when do muds, sands, and lime deposits that are 

 permanently under a water cover become cemented into solid rock? 

 Calcareous muds back of the Florida Keys, and up to 20 feet thick, are 

 still soft and rest uncemented on the hard and eroded Pleistocene lime- 

 stones. It was out of these muds that the "railway over the seas" was 

 built. It is therefore evident that some marine deposits may remain 

 unconsolidated for thousands of years, and yet in some of the Ordovician 



