44 



PRINCIPLES OF STRATIFICATION, 



broken in the line of' their rise, and the same stratum may crop out 

 in one place, and appear again, farther on, in the line of its rise, as 

 represented Plate I. fig. 2. We must be particularly attentive to this 

 circumstance, otherwise we may commit the most egregious errors 

 in describing a country which we have travelled over, where there is 

 no opportunity of seeing a section of the strata. Thus, in fig. 2., 

 after passing over the beds 1, 2, 3, 4, and having no easy method of 

 ascertaining the dip, we may, without great care, mistake the beds 

 4, 3, 2, 1, as different and lower beds in the series. Ebel and ma- 

 ny flying geologists have made this mistake. In some instances we 

 come suddenly to the termination of a whole series of strata, as in 

 descending the Cotswold Hills into the Vale of Severn ; the lime- 

 stone called Roe-stone, of which they are principally composed, is 

 not found on the other side of the valley, nor in any part of Eng- 

 land to the north-west of it. Has this limestone ever extended far- 

 ther westward ? and if it has extended farther, by what cause has 

 it been removed ? These enquiries will be adverted to in a following 

 chapter. 



To return to our pasteboard planes, arranged as before described, 

 with the edges rising from under each other in the conformable posi- 

 tion. If we take another series of planes, and lay them flat over the 

 outcropping edges of the conformable series, we shall then have the 

 unconformable position represented, Plate I. fig. 3. Now, the stra- 

 ta that cover the lower stratified class in England occur in this posi- 

 tion ; and the following important inference may be drawn from it, 

 namely, that the under stratified rocks had been formed, and their 

 strata broken and raised up, at a period which must have preceded 

 the formation of the upper series, by a considerable interval ; for, the 

 lower series were evidently solidified, and, afterwards, in many in- 

 stances broken, and the fractured edges of the strata levelled, before 

 the upper strata were deposited upon them. 



The most common error which persons commencing the study of 

 geology are liable to make, is in mistaking the apparent for the real 

 inclination of the strata. Plate I. fig. 4. will render this more intel- 

 ligible than any description. It represents a portion of a stratified 

 mountain, of which the strata have a considerable dip to the east. 

 If the escarpment or section be made in the line of bearing, C D, 

 the strata will appear to range from north to south, without any rise 

 or dip, and would be described by a young observer, as being hori- 

 zontal. But if an opening or section be made, on the side parallel 

 to the line of dip, as at C C, the true inclination will be seen. Any 

 section, made in an oblique direction to the line of dip, will cause 

 the inclination to appear less than the true one, and the line of dip 

 will appear to vary from the true dip. The chances, therefore, are 

 very great against the natural section made in a mountain presenting 

 the true dip and inclination of the strata. Another error which a 

 person who does not attend to the dip and direction of the strata may 



