234 ON THE DETRITAL TIN-ORE OF CORNWALL. 
sequently, a less rapid course ; and—as they receive the drainage 
of wider tracts—their volume is proportionally larger. In steeper 
parts of their progress, and where they traverse rocks in some 
state of disintegration, however, they erode and carry with them 
certain portions of the débris; but, where the declivity diminishes, 
the velocity—and with it the suspending power—of the current 
declines; and some, at least, of the suspended matter subsides* 
to the bottom or on the margin of the streams. Where the 
rivers fall into broad gulphs—as at the Mount’s-bay and the bay 
of Tywardreath, or into deep inlets—as at Falmouth and Fowey, 
—their mouths are rarely obstructed by shallows. But the bars 
of sand,. which to a greater or smaller extent—obstruct the 
entrances to all our northern creeks,{ can scarcely be said to have 
assumed their normal forms, if, indeed, their actual positions, under 
action of the sea alone; for they are barely uncovered, by the 
ebbing tide, before the finer particles begin to drift, under the 
influence of even an ordinary breeze.{ During heavy gales{ the 
sands rise in clouds, and—driven far inland—irreclaimably over- 
whelm large tracts of country ;§ of late years however, such 
* Thomas, (R.), History of Falmouth, p. 46. 
+ Borlase, Natural History, pp. 44-47. De Luc, Geological Travels, iii, 
pp. 178, 181, 182-3. Paris, Cornwall Geol: Trans: i, p.4. Boase, Ibid, iv, p. 
260. De la Beche, Report, pp. 425-428. Sedgwick and Murchison, Geol: 
Trans: v, (N.8.), p. 285. 
~ “Saint Peran...too well brooketh his surname, in Sabulo: for the 
“light sand, carried up by the north wind, from the sea shore, daily con- 
‘‘tinueth his covering, and marring the land adjoynant...draue the Inhab- 
“itants to remooue their Church.” Carnw, Survey of Cornwall, f. 148. 
‘My father has informed me that within seventy or eighty years, a field 
‘in preparation for tillage, by some of his relatives, in Perran-zabuloe, was, 
‘“¢during a storm, irreclaimably covered in a single night with sand, which 
‘had drifted to the depth of a foot.” Hrnwoopn, Cornwall Geol: Trans: v, 
p. 112, Note +. 
§ Daily observations on the winds which prevailed at Penzance during 
twenty-one years (1807-1827) show their annual average directions to have 
been— 
Wooscodod S00000000 50 days. Shs codoo00d0SGodos 41 days 
Iolo Goodoosc0s00 2 SbiW “bascoocaDecs ay es 
IDG s00 000605000000 AUS 55 Wiois6s500d000030 123 .,, 
SuldG coco b000000000 DY op INS W.. ets crescent 146 
” 
Gippy, (H. C.), Phil: Mag: and Annals, iii, (1828), p. 179. 
