376 Prof. NECKER on the Determination of the 



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place the protractor vertically on the N. b. W. line, for instance, 

 and to raise the half ring till it should touch the 45th degree ; 

 finally, to place the protractor on the index line, indicating the 

 rise to be towards the W.N.W., and to note the angle 60. 



We hope that this simple little mechanical contrivance will 

 be of use to the practical geologist, who may not have the means 

 or leisure to use the more accurate modes of determination 

 before described. The habit which he will acquire of giving 

 his attention to observations connected with this subject of in- 

 quiry, will enable him not only to ascertain in all cases the exact 

 position of the strata whenever they are real planes, but also to 

 discover when two distinct sets of strata are lying in the same 

 plane, and if not, to recognise modes, hitherto little attended to, 

 of unconformable stratification. 



Finally, although all the modes of determination above allud- 

 ed to are only appropriated to the plane strata, the readiness of 

 tact and eye-sight which such considerations will have given to 

 the geologist, will not be without use, when he shall have to 

 study the waving disposition of strata with curved surfaces, whe- 

 ther they are parts of parallel portions of cylinders with their 

 axes horizontal, in which case the seams parallel to these axes 

 or the direction will also appear horizontal, as is observed in that 

 part of the Lammermuir Hills which forms the coast of Berwick- 

 shire ; or whether they are parts of oblique cylinders with their 

 axes inclined to the horizon, in which case there is no seam which 

 can appear horizontal, because there is a dip even in the direc- 

 tion of the bearing. Of this case the Alps show an amazing va- 

 riety of instances, among which the more or less dismantled ob- 

 lique cylindrical structure of the Mount Saleve near Geneva, and 

 of the whole mountain group on the southern bank of the Arve, 

 between Bonneville and Sallenches, are the most instructive and 

 worthy of remark. 



