PRINCIPLES OF STRATIFICATION. 



43 



bearing. The angle at which the stratum rises-above the horizontal 

 line or level is called the Inclination, Suppose the western edge of 

 the pasteboard plane is raised above the table, forming with it an an- 

 gle of thirty degrees ; then, we say the direction of the stratum is 

 north and south, its dip east, its rise of course west, and its angle of 

 inclination thirty degrees. Simple as this appears, geologists of con- 

 siderable eminence have made the most palpable mistakes, in defin- 

 ing stratification. It has been said correctly, that, the line of dip be- 

 ing, always, at right angles to the direction or line of bearing, when 

 the dip is given, the direction is known : but when it is further said, 

 that, if the direction is given, the line of dip is given also, the asser- 

 tion is erroneous ; for let the above plane of pasteboard be again laid 

 flat upon the table in the same direction, due north and south ; and 

 instead of tilting up the western edge, if we tilt up the eastern we 

 shall then have the same line of bearing as in the first instance, but 

 the dip will be west instead of east. 



It sometimes happens that a stratum, without varying its direction, 

 may be so bent as to dip two ways in the same mountain, like the 

 sloping sides of the roof of a church, or the letter V reversed (a). 

 (See Plate I. fig. 2. stratum 4. and 5.) Place the two planes of 

 pasteboard in a north and south direction, and raise them so as to 

 make the upper edges meet; we shall then have the line of bearing 

 north and south as before and the dip east on one side and west on 

 the other. The limestone strata at Dudley Castle Hill dip on each 

 side of the hill as above described. (See Plate III. fig. 4. B.) 

 When strata are bent on each side of a mountain, without being 

 broken at the top, they are called saddle-shaped. A line traced on 

 the surface of a country, to designate where the strata dip in oppo- 

 site directions, has been called the anticlinal line, and should be in- 

 troduced in all geological maps, when it can be conveniently ascer- 

 tained. 



Whatever may be the inclination of a stratum, its true thickness is 

 measured by a line perpendicular to the upper and under surface. 



If we take a number of similar planes of pasteboard of different 

 colours, and lay the undermost a litde inclined, and place another 

 plane upon it, v/ith the upper edge about an inch or more distant 

 from that of the under stratum, and again lay the others in succes- 

 sion in the same manner ; the uncovered ends of the planes will 

 rise from under each other, like a number of slices of bread laid 

 on a plate. These uncovered edges will represent the outcrop or 

 crop of the strata, and it will be perceived how we may obtain a 

 knowledge of an under stratum without sinking or boring, merely by 

 crossing a country in the line of the rise or dip of the strata. When 

 strata are arranged in this manner, they are said to be in a conform- 

 able position. (Plate I. fig. 1.) It will naturally be enquired wheth- 

 er the strata absolutely terminate where we find their outcrop. In 

 some instances this is the case ; but frequently the strata are bent or 



