260 W. A. E. Uss/ter — Pleistocene Geology of Cormvatt. 



Corn. vol. iii. p. 332) mentioned the occurrence of alluvial soil 6 to 

 9 feet in thickness on the shelf, and at Numphra Moor not exceed^ 

 ing 5 feet. 



10. Mr. Henwood (J. E. Inst. Corn. vol. iv. p. 199) gives a 

 section of the bed of a rivulet at St. Erth, near Hayle, as follows, the 

 thickness of the deposits not being given : Gravel, sand, and mud, 

 on peat, under which roots, trunks, and branches of trees, with 

 quantities of mud, were found resting on tin ground, poor and not 

 extensive. 



Mr. Carne (T. E. G. S. Corn. vol. iv. pp. 105-111) gives the 

 following general notes on Diluvial tin. Cap shelves are tabular 

 masses of rock projecting from sides or bottom of the tin ground, so 

 as to allow of the occurrence of tin ore under them. Copper, not 

 found in tin gravels, probably because rarely so near the surface 

 as tin, and in the form of sulphuret so liable to decomposition. The 

 traces of gold met with were probably derived from undiscovered 

 veins on the east. All the productive streams occupy valleys 

 opening on the S. coast, whilst most of the richest tin veins are 

 near the N. coast. The direction of the tin streams seems to have 

 been from N.N. W. to S.S.E. In narrow valleys little tin ore is ob- 

 tainable. In steep valleys all the ore is upon the shelf. In very 

 gently sloping valleys tin ore is met with to within two or three 

 feet of the surface, as at Chyanhall. In gently sloping valleys the 

 tin ground is thick but poor, owing to admixture with alluvial 

 sediments. 



General Notes. 



As the stream tin gravels were deposited during the last stages in 

 the elaboration of the present drainage system, their watershed 

 boundary can scarcely have differed much from the present; it is, 

 therefore, only natural that, whilst the richest tin veins are near the 

 north coast, the most productive streams occupy valleys opening on 

 the south coast. 



The position of the tin ground with reference to the sea-level in 

 the estuarine sections is, unfortunately, seldom given. In Mr. Hen- 

 wood's section on Marazion Green (8 A), mention of overlying alluvia 

 seems to have been omitted ; as Mr. Carne, in a section at Huel 

 Darlington, near Marazion Eiver, gives twelve feet of peat and gravel 

 above the sea sand, and the surface is given in Mr. Henwood's 

 section at twenty to thirty feet above the sea-level, the top of the 

 marine bed would appear to be a few feet above high water (Carne, 

 T. E. G. S. Corn. vol. vi. p. 230). 



Again, in Mr. Smith's section (5 G) of Lower Pentuan, the shelf is 

 said to be at low-water level, which would place the top of the 

 upper marine bed at about forty feet above low water, which, con- 

 sidering the absence of marine deposits at Wheal Virgin Works (5 B) 

 and Upper Pentuan (5 D), is out of the question; so that either the 

 thicknesses are not given in feet and inches, or the level of the shelf 

 is erroneous. 



Mr. Carne (T. E. G. S. Corn. vol. iv. p. 47) describes the tin 

 ground of Drift Moor Works, near Newlyn, as resting on the sides 



