ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 65 



In the West of England the only important examples of 

 such beds are the slightly auriferous tin-gravels, which, origin- 

 ating for the most part in the granitic uplands, formerly occupied 

 the lower portions of many of the valleys, though buried under 

 still more recent deposits devoid of tin. 



We may say now, as Mr. Henwood said nearly 20 years 

 ago, that, having been wrought from remote antiquity they are 

 now nearly, but not quite exhausted. In particular, such gravels 

 are known to exist unwrought in the seaward extensions of 

 several of the valleys on the south, and in one instance* an 

 extensive engineering operation was undertaken to open up such 

 a deposit, but the project did not result in a financial success. 



The detrital tin-deposits of Cornwall have been very 

 admirably described by a host of observers, among whom may 

 be particularly mentioned the names of Colenso, Budge, Prideaux 

 Carne, and Pattison; but especially Mr. Wm. Jory Henwood, 

 whose Presidential address to the Eoyal Institution of Cornwall, 

 in the year 1873, dealt exhaustively with the subject. Mr. 

 Henwood described with more or less of detail, and from his own 

 personal observations, more than 20 different stream works, and 

 gave many references to the descriptions of previous writers. f In 

 all of these the richest " tin-ground " was found resting directly 

 upon the solid rock beneath, the " shelf," or " bed-rock." This 

 was usually covered with several alternating layers of peat or 

 vegetable-mould and sand or gravel. Sometimes an upper and 

 poorer layer of tin-ground occurred resting on a "false-shelf," 

 not far beneath the surface, the components of this upper layer 

 being, as Mr. Henwood observes, generally less completely 

 rounded than those of the true tin-ground. He also observes, 

 that the components of the tin-ground generally are more 

 rounded in proportion to their distance from their elevated 

 sources ; and furthermore, that the more elevated tin-grounds 

 resemble in mineral character the rocks upon which they rest, 

 while those nearer the mouths of the valleys have no such 

 resemblance. 



* At Eestronguet, about 20 years ago. See Taylor, Proc. List. Mech.Eng., 

 1873, p. 175. 



f See Jour, Roy. Inst. Corn,, 1873, 



