68 CHARACTERS OF STRATA 



Of the Forms of Strata. 



The most perfect form of a stratum, is that in which 

 the two planes are accurately parallel, but it is the 

 most rare. They are more commonly inclined in dif- 

 ferent ways ; so that a bed terminates at length, in 

 one or more directions, or in all, by a thin edge ; 

 while it may also present surfaces so frequently and 

 unequally inclined or undulated, as to be of various 

 degrees of thickness throughout. 



The thickness of a stratum may vary from one of 

 many yards to that of paper ; and it is obvious that 

 the thinner cannot easily be very extensive. The 

 extent of surface which any one may cover is equally 

 various : it may amount to many miles ; but, in these 

 cases, it is traced rather by comparing detached parts, 

 than by a continued view of the whole. That com- 

 parison is made by means of the consistent mineral 

 nature of all the parts, the resemblance of the organic 

 contents, where these are present, the correspondence 

 and nature of the other strata with which they are in 

 contact, and the similarity of position which they 

 possess towards the perpendicular. 



Thus it is anticipated that a stratum may be inclined 

 to the horizon; but, in fact, they are rarely quite 

 horizontal. The deviation from the horizontal posi- 

 tion constitutes the inclination of a stratum ; and the 

 true inclination is evidently the greatest angle which a 

 line ta"ken on that plane forms with the perpendicular; 

 the dip implying, further, that point of the compass 

 towards which it is directed. As, in consequence of 

 the inclination of a stratum, its edge must somewhere 

 appear at the surface of the earth, an imaginary line 

 lias been contrived to represent it, called the direction 

 of the stratum, drawn at ri^ht angles to the line of 



