42 



PRINCIPLES OF STRATIFICATION. 



by travelling over the strata from a to 6, we come upon the outer 

 edges 1,2, 3, and may trace their order of succession, as they rise 

 from under each other. In ravines and the escarpments of moun- 

 tains, and in the cliffs on the sea-coast, we are also enabled to trace 

 the position and order of succession of rocks. But, to do this with 

 tolerable correctness, we must have an accurate knowledge of strati- 

 fication in all its various possible forms. However simple the prin- 

 ciples of stratification may, at first, appear, this knowledge, when ap- 

 plied to practice, is not of such easy attainment as some may ima- 

 gine ; and for want of it, geologists of considerable eminence have 

 fallen into the most egregious errors. A knowledge of stratification 

 is, indeed, of far greater importance to the practical geologist, than 

 an acquaintance with the minutiae of mineralogy or conchology. 



Though the word Stratum, in its original language, and by gener- 

 al acceptation in speaking of rocks, denotes a bed, it is convenient 

 to restrict the term bed to a stratum of considerable thickness; for 

 such beds are often subdivided into several distinct minor strata, and 

 we cannot well describe a stratified stratum. 



When a series of strata of a similar rock are arranged, with occa- 

 sional strata of rocks of another kind intervening which recur in dif- 

 ferent parts of the series, they are regarded as having been all form- 

 ed, nearly at the same epoch, and under similar circumstances; and 

 such series are called, by geologists, Formations. Thus, the strata 

 of shale, sandstone, and ironstone that accompany beds of coal are 

 called the Coal formation. Strata of different kinds, in which a 

 gradation into each other is observed, and which contain similar spe- 

 cies of organic remains, also constitute a geological Formation. The 

 chalk with flints, the lower chalk without flints, the chalk-marl and 

 the green sand under the chalk, are regarded as members of what 

 is denominated the Chalk formation. The student, however, must 

 be careful to distinguish the different meaning of a rock formation^ as 

 here described, and the formation of a rock : the latter term implies 

 the mode of formation, or the agent by which the rock was formed 

 or consolidated ; whether by igneous fusion, as beds of lava; by de- 

 position from water, as beds of clay and sandstone ; or by animal se- 

 cretion, as beds of coral. 



In order to obtain a distinct idea of stratification, in its simplest 

 form, let the young geologist take a piece of pasteboard or thin 

 wood, say 12 inches square : let him divide it in the middle into two 

 equal planes, each 12 inches in length and 6 in breadth. Place one 

 of these planes flat on a table with the ends facing the north and 

 south ; the sides will of course be at right angles, and face the east 

 and west. Now, if one of the sides be tilted up, — say the western 

 side, — we may suppose the pasteboard plane to represent a stratum, 

 rising to the west and dipping eastward. The lengthwise direction 

 of the plane is called the line of hearing; and the declining direc- 

 tion is called the line of dip, which is at right angles to the line of 



