U18 E. BLACKWELDER CHARACTERISTICS OF CHEMICAL DEPOSITS 



tliey are rare even in the marine deposits. It is no nniisnal experience 

 for the geologist to chance on a formation, 5,000 or more feet thick, that 

 is devoid of fossils ; or if fossils are found they may not he indigenous, but 

 imported, like the battered logs in the Triassic red beds of Arizona; or 

 they may owe their presence to temporary invasions of the sea, like the 

 marine limestones in the Pennsylvania coal measures. It is true that cer- 

 tain types of sediments, such as peat and saline beds, are made only on 

 land, whereas such deposits as greensand and coral limestone are confined 

 to the ocean ; but these are not the most common deposits represented in 

 our sedimentary rocks. Certain varieties of current-marks and cross- 

 bedding may be limited to continental deposits. Sun-cracks are absent 

 from marine deposits except the extreme littoral and are rare even there. 



After all these general criteria for distinguishing marine and conti- 

 nental sediments have been taken into account, however, it must not be 

 forgotten that most of them occur but locally, and that in many deposits 

 all of them are lacking. It is hardly stretching the truth to say that most 

 unfossiliferous strata require a critical analysis of various characteristics 

 before they yield a satisfactory answer to the question of their origin. 



In tlie modern ocean more than one-third of the deposits (areally) are 

 non-clastic and nearly half are comprised in the unique abysmal red clay; 

 on the continents, however, more than 95 per cent are clastic. The rea- 

 sons for this contrast are too obvious to demand further mention. 



Among tlie clastic deposits of the continents there is a wide range of 

 N'ariation within which nearly all possible intergradations may be found. 

 On the whole there is far more difference between extreme types of conti- 

 nental elastics, such as till on the one hand and dune sand on the other, 

 than between such a continental deposit as lake marl and such a marine 

 sediment as calcareous mud. It is partly for this reason that no one cri- 

 terion serves to distinguish the two general classes from each other. 



Determining Factors 



To bring order out of this seeming chaos and to present briefly the 

 leading characteristics of the continental sediments, it is necessary to 

 analyze and classify the sediments into distinctive kinds. We may first 

 consider the factors which control the formation of these individual kinds 

 The four principal factors seem to be : 



(a) the lithology of the parent rocks, 



(h) the processes of weathering at the source, 



(c) the processes of transport and deposit, 



(d) the conditions at the place of deposition. 



