42 M. Mohs' Summary of Geogtiostical Phenomena. 



clivities and in the valleys. Particularly striking eminences, 

 whatever form they may possess, generally exhibit a composi- 

 tion different from that of their vicinity, and are therefore 

 worthy of notice ; and on the other hand, trough-shaped hol- 

 lows, basin-like widenings of valleys, contractions of valleys, and 

 similar phenomena, are often serviceable in enabling us to 

 draw useful conclusions. In order to apprehend at one glance 

 the necessity of the accurate consideration of the superficial 

 relations of a mountainous tract, which is in the course of in- 

 vestigation for the discovery of useful minerals, we have only 

 to reflect that the external form is founded on the internal 

 constitution of the district, and is the consequence of the uni- 

 form operation of a general cause. 



***** 



Most mountainous tracts possess so great an extent, that an 

 investigation like that before us can with difficulty be extended 

 over the whole at the same time, and hence, in most cases, it 

 must be confined to a particular portion. We have not only 

 to obtain a knowledge respecting this particular part as re- 

 gards all the features already mentioned, but also to inform 

 ourselves fully as to its relations to the whole, and to ascertain 

 sufficiently its position relatively to the high mountain-ridge 

 and the extent it occupies of the mountain-ridges which pro- 

 ceed from it. It is not only useful, but in many cases neces- 

 sary, to go over, with sufficient minuteness, the neighbouring 

 ridges to a certain extent, and also the valleys belonging to 

 them, or by which they are separated from the particular 

 district under investigation, in order by this method, either 

 through the means of already existing mining operations, or a 

 particularly good opportunity, to institute observations, to ob- 

 tain indications, and to direct attention to objects, which, with- 

 out it, might probably have been overlooked. It may be remark- 

 ed generally, that it is very advisable, before the commencement 

 of the actual examination of a particular district of a moun- 

 tainous tract of country, to make ourselves as fully acquainted 

 as possible with the whole existing mining operations, while 

 these reveal a series of phenomena, regarding which we can, 

 even in the most fortunate circumstances, acquire a knowledge 

 of only the first member, by the study of the external rela- 



