5i 



pressure, when frozen, or by the pounding action of waves or water- 

 falls. The sawing back of waterfalls is an example of erosion, 

 which only takes place when the water carries solid particles of 

 sand, pebbles, etc. Pure water has no power of erosion, only of 

 washing away debris. Nearly all valleys have been produced by 

 rivers and streams. In very dry regions, as in Colorado, they tend 

 to be deep with nearly perpendicular sides ; where there is more 

 rainfall they occur with sloping sides. The Llanberis Valley is a 

 good example of tributary streams modifying the sides. Water not 

 only denudes away rock, but re-deposits the material elsewhere. 

 Rivers and streams thus tend to fill up valleys in one part, where 

 the current is slow, whilst carving them out in higher parts, where 

 the currents are more rapid. Along our coasts the waves act by 

 cutting back the land horizontally, leaving what afterwards becomes 

 a " plain of marine denudation." Hard masses of rock resist this 

 action and remain as " stacks," such as the Old Harry, near 

 Swanage. The sea washes softer rocks away, forming bays, whilst 

 the harder rocks resist its action and remain as headlands. 



Denudation by Chemical Action of Water : — This is caused by 

 rain water becoming acid, through absorption of carbonic acid gas 

 from the air, slowly dissolving and eating away certain rocks. 

 Granite is decomposed in this way, and its constituents re-deposited 

 elsewhere, the quartz as sand and the felspar as china clay. The 

 granite Tors of Dartmoor have been carved into the present rugged 

 forms chiefly by this agency. But limestone is the rock most easily 

 affected by this process ; the great caverns of Derbyshire, Cheddar, 

 etc., and many of the great valleys in limestone districts have been 

 primarily produced by the dissolving out of the lime by water, and 

 its re-deposition often as stalactite. 



Denudation by the Mechanical Action of Water : — This occurs in 

 two ways : (i) by the weight of water hurled at rocks by sea waves 

 or waterfalls, and (2) by the splitting asunder of rocks through the 

 expansion of ice in their cracks and crevices. This occurs exactly 

 as our water pipes burst in winter after freezing, and has been one 

 of the most potent agents employed in carving out rock scenery. 



There are two minor denuding agents which act independently 

 ot water that should be mentioned. First, expansion and contrac- 

 tion of rocks in countries having a very hot day, but a cold night 

 temperature, which gradually produces splits and fissures ; secondly, 

 wind erosion in dry climates and deserts caused by sand whirled 

 round in wind storms gradually grinding down out-standing rocks, 

 which often assume curiously fantastic shapes. The celebrated 

 Brimham rocks, near Harrogate, were so produced. 



At the second demonstration, on February 15th, 



Basin "fts * this sub J ect was considered. A "basin" is, 

 q 3 . j n * s geologically, a " sag " or depression in the earth's 

 crust, bounded by ranges of hills or mountains. 

 The Hampshire basin is bounded on the north by the South 

 Downs, a range of chalk hills, and, and on the south by the Isle of 

 Wight ridge and the Purbeck Hills, broken into by the sea in 



