464 APPENDIX. 



racy in no other way. He who produces a map from 

 his journal, cheats himself as well as his readers : of 

 maps compiled from the notes of others, I can only- 

 say that they might as well not have existed, if there 

 he any meaning in the term map, or any purpose in 

 geological topography : while I should insult the most 

 ignorant reader, by a remark on the attempt to pro- 

 duce such a map from collected specimens. 



For verifying positions, a portable compass is ge- 

 nerally sufficient: and it should be elevated on a staff, 

 partly for convenience, and partly to avoid the effect 

 of local magnetic influences, far more common than 

 is suspected, especially in districts of trap. For more 

 minute observations, the two-inch sextant is peculiarly 

 convenient, where all must be portable ; while the ne- 

 cessity for a spirit-level cannot often occur. If com- 

 plicated and balky machines have been invented for 

 taking the elevations of strata, their weight is not the 

 only objection ; since these cannot be discovered from 

 single, or even from many, partial observations. Few 

 strata present surfaces sufficiently level, or so coinci- 

 dent with the general plane, as to permit of a true 

 measurement through the base of these quadrants; as 

 many rocks also are inaccessible. But as the profile 

 can generally be seen, a small ivory quadrant of two 

 or three inches radius, or, for a less accurate eye, a 

 similar one which I have constructed from mica, will 

 serve to determine the angle, by prolongation or co- 

 incidence ; and with an accuracy as great as from par- 

 tial observations through other instruments, whence, 

 after all, an average must be deduced. Nor can a 

 tolerable eye require an appended plummet to deter- 

 mine the perpendicular ; though this also is easily ap- 

 plied. And this simple instrument is equally appli- 

 cable to a tangible stratum, by means of a straight 



