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University of California Publications in Geology [Vol. 7 



The production of geologic bodies by these processes is brought 

 about by the settling, precipitation or spreading out of the com- 

 ponent materials on some preexisting basement or floor and the 

 building up of the mass by the addition of fresh material 

 normally to the upper surface of the deposit. The basement or 

 floor may be the basin of a sea or lake, the bed of a stream, the 

 surface of the ground, more rarely the floor of an underground 

 space or cavern, etc. The appearance of layer-structure is caused 

 by the varying nature or intensity of the agents supplying the 

 material or by the sorting action of the medium or media through 

 which the material passes before coming to rest. The distinction 

 between layers may therefore depend on differences in composi- 

 tion, size of grain, or perhaps only on color change, or on the 

 occurrence of thin bounding layers or separation planes between 

 tabular masses of the same nature. The thicker layers are gen- 

 erally referred to as strata or beds and the thinner as laminae. 



Stratification as above defined is geologically a very important 

 condition. It indicates an essentially superficial or epigene origin 

 of the formations exhibiting it, and except for the lava sheets 

 which are set apart by a special group of characteristics, is a 

 distinctive structure common to all of those closely related 

 processes which are grouped under the general name of sedi- 

 mentation. Furthermore, chiefly as a result of gravity control 

 in these processes, the separation planes between the layers — the 

 stratification planes — are in most types of these deposits formed 

 horizontally or at only a slight inclination to the horizontal. 

 This is a fundamental datum for structural geology, for with it 

 as a basis we may judge of the nature and amount of earth 

 movements that have tilted, folded, and otherwise disturbed the 

 stratified formations. 



Pseudostratification. — In this paper the waiter will describe 

 occurrences in which there is a layer-structure in clastic sedimen- 

 tary rocks giving the appearance of beds and often laminae, but 

 not produced by the processes that brought the rock masses into 

 existence. Furthermore, these structures have been formed both 

 with horizontal attitude and with considerable inclination to the 

 horizontal without relation or reference to the amount of tilting 

 that has been suffered by the formation in which they are found. 



