364 Prof. NECKER on the Determination of the 



or seams, as corresponding to the true inclination of the plane of 

 the strata, would be entirely misled ; and if, from this supposed 

 inclination of the plane, he wished to infer the bearing of the 

 strata, he would meet only with the most inextricable confusion, 

 instead of perceiving that remarkable regularity in the direction 

 of the strata which is so conspicuous in mountain-chains, what- 

 ever may be their extent. 



SAUSSURE, in his Agenda, has already cautioned the geologist 

 against too hasty an inference from the horizontality of the lines 

 of stratification on the surface of a precipitous crag : he has 

 shown that, before pronouncing a mountain to be formed of hori- 

 zontal strata, it is necessary to view it from the extremities of 

 two lines crossing each other at an angle. 



The following considerations will show that SAUSSURE'S ob- 

 servation is not applicable only to the case on which the seams 

 of the strata, when observed from only one side of a mountain, 

 appear horizontal, but that it applies equally well to inclined lines 

 of stratification, which make with the horizon an angle smaller 

 than the true angle of dip of the planes. 



Let us suppose a system of parallel planes or strata more or 

 less inclined, into which a section should be made by a vertical 

 plane, whose direction or line of common intersection with the 

 horizontal plane would be parallel to the direction of the strata 

 themselves. In such a case, the seams of the strata on the sur- 

 face laid bare by the section would appear horizontal. If, on the 

 contrary, the direction of the vertical section was supposed to be 

 perpendicular to the direction of the strata, then the seams of 

 the strata would exhibit a dip identical with the true dip of the 

 planes themselves. But between these two limits an indefinite 

 number of vertical sections may be supposed, in which the angle 

 of dip of the seams will vary according to the direction of the 

 section. The more this direction approaches to a parallelism with 

 the direction of the strata, the lesser will be the angles made by 



