698 



PRINCIPLES OF STRATIGRAPHY 



cation being due to change in material, to change from one type of 

 clastic to another, or to alternation of clastic with endogenetic de- 

 posits. The most typical development of stratification is in the 

 water-laid elastics (hydroclastics), and especially in those laid down 

 in standing water. Before considering the various kinds of stratifi- 

 cation, however, we must first have a clear conception of the mean- 

 ing of the term stratum. . 



Definition of Stratum. The current definitions of the term 

 stratum vary greatly, as will be seen from the following quotations : 

 (a) "A layer of rock; a portion of a rock mass which has so much 

 homogeneity and is so separated from the rock that lies over and 

 under it that it has a character of its own." (Century Diction- 

 ary.) (b) "The term stratum is sometimes applied to one layer 

 and sometimes to all the consecutive layers of the same sort of 



Fig. 135. A llul'^s oi su.iutitd locks 

 bounded 1)\ joint faces and iso- 

 lated by erosion. The strata are 

 inclined but appear horizontal in 

 one section. 





Fig. 136. Stratified chalk penetrated 

 by pipes of sand and gravel. 

 Kent, England. (Prestwich.) 



rock." (Chamberlin and Salisbury, Geology-i : 464.) (c) "Strata 

 or Beds are layers of rock varying from an inch or less up to many 

 feet in thickness. A stratum may be made up of numerous laminae, 

 if the nature of the sediment and the mode of deposit have favored 

 the production of this structure . . . [it] . . . may be one of a 

 series of similar beds in the same mass of rock, as where a thick 

 sandstone includes many individual strata, varying considerably in 

 their respective thicknesses ; or it may be complete and distinct in 

 itself, as where a band of limestone or iron stone runs through the 

 heart of a series of shales." 



"The smallest subdivisions of the Geological Record are lam- 

 inae, a number of which may make a stratum, seam or bed. As a 

 rule, a stratum is distinguishable by lithological rather than palae- 

 ontological features." ( Geikie-Textbook, 4th ed.-i:(5j5; 2:860.) 



(d) "In the description of a formation the term stratum (from 



