Notices of Memoirs — W. J.Kenwood — Detrital Tin-ore. 177 



V. — Observations on the Detrital Tin-ore of Cornwall. By W. 

 J. Henwood. Journal Boyal Institution Cornwall, No. xv. 1874. 



THESE deposits in Cornwall and Devon have been worked for 

 ages, and are now nearly exhausted. This paper refers chiefly 

 to those few still in progress. 



Between the Land's End and St. Ives the granite and slate are 

 crossed by lodes, differing in direction and character in different parts. 

 To the south they are poorer, in the central tract richer, and in one 

 spot tin-ore is generally disseminated ; to the north and north-east 

 many rich lodes have been and are largely worked, sometimes long 

 irregular " off-shoots " spring from the lodes, and at one place im- 

 mense metalliferous masses occur in the granite. In this tract tin- 

 ore has been the chief product, though other ores have been worked. 



At Bosworlas a narrow strip of " tin-ground " (granitic gravel, 

 with pieces of tin-stone, etc.) is worked, at a depth of about 3ft. from 

 the surface, having a sea-ward slope down the ravine, and varying 

 much in thickness (Sin. to 30in.) from the uneven surface of the 

 granite beneath. 



Near Bejowans the tin -ground occurs at a rather greater depth 

 below gravel, sand, and peat. Other places give like sections, and 

 generally there is an uneven surface of granite beneath. 



Around the Camborne gi'anite-tract there are lodes with tin-ore, 

 especially near the southern boundary, and the streams crossing 

 these parts are still worked for tin, the sections being of the same 

 kind as before, and the detrital matter sometimes resting on the out- 

 crop of tin-bearing veins in the granite. Near Tregedna the stream 

 tin-deposit is overlaid by from 20ft. to 30ft. of mould and hardened 

 silt. The largest body of stream-tin wrought of late years is near 

 Eestrouguet-creek, not only by open works, but by shafts 50 or 

 60ft. deep, and drifts below the water. 



In the central district (Hensbarrow) tin-ore occurs not only in the 

 lodes, but is sometimes scattered through the granite (St. Austell), 

 and the detrital deposits on the north and east are still worked. 

 Three sections, of varying depth, near St. Austell, show like deposits 

 (with peat) to those noticed above, and there are others close by. 

 On Tregoss Moors, which are on the slate, often soft, and with 

 *' elvans," three shallow sections are still open. 



The eastern district includes the great granitic range, the highest 

 part of Cornwall. The granite, slate, elvan, and hornblendic rocks are 

 all crossed by lodes and cross-veins. Tin-ore occurs in some of the 

 lodes, and also in scattered masses. Near St. Neot stream-tin is worked. 



The water-shed of Cornwall has several flexures, but its range is 

 nearly always much nearer to the north coast than to the south ; 

 consequently the shorter and swifter brooks on the north have 

 greater wearing and carrying powers than the longer and slower 

 streams on the south, and the former, therefore, deposit very little 

 matter in their beds, whilst the latter deposit more. 



The debris of the bed beneath the tin-ground always prevails in 

 it. Other ores than tin-ore occur. The proportion of tin-ore in one 

 of the richest works scarcely exceeded that in one of the poorest 



DECADE II. — VOL. I. — NO. IT. 12 



