22 PROCEEBINGS OF TKE GEOLOGICAL SOCIEXY. [Mar. 24, 



falling to decay. Although in the habit of inspecting the basins in 

 every state of the weather, from the mildest breeze to the heaviest 

 storm, the Author had never seen these particles blown about in 

 the water in the basins having the bottoms flat and sides upright, 

 and had only seen them moved in shallow concave basins when dry, 

 or when a heavy gale had blown them out together with the water. 

 The cause suggested by Dr. M'CuUoch could not affect the deep basins, 

 as in those cases the particles would be undisturbed by motion of the 

 water from wind. These small fragments, however, throw some light 

 on the manner of the formation of the rock-basins. The granite of 

 the Dartmoor district is in a great measure porphyritic ; it is for the 

 most part of a large coarse grain, and schorl in variable proportions 

 frequently occurs ; globular nodules varying from an inch to upwards 

 of a foot in diameter, often occur. These vary much ; sometimes they 

 are harder than the adjoining rock, sometimes scarcely coherent, 

 and soon falling away on exposure to the weather. Along the 

 belt where the basins exist, the granite is for the most part more 

 liable to decomposition than at the harder and more crystalline Tors. 

 This is shown by the many rounded Tors, and every roadside cut- 

 ting shows the rapidity of the decay. The division of the granite 

 into tabular sheets of rock of irregular thickness, causing the ap- 

 pearance of stratification, is common to all the granite of this district. 

 In irregularities on the surface of the granite, and in hoUows, very 

 probably in many cases caused by the nodules above noticed, water 

 lodges and penetrates the porous granite, and the decay thus com- 

 menced will gradually enlarge the cavity to a basin. During the in- 

 clement part of the year these basins are full of water, that during 

 part of the time often rapidly alternating with ice. When the warm 

 weather comes on, the water evaporates, and the basins are dried 

 up ; from the frequent showers there is, then, a constant change 

 between the rock being saturated with wet, and being warm and dry. 

 The gradual action of the water is very perceptible ; when it has 

 evaporated, the stone up to the water-line is left a lighter shade 

 than the adjoining rock ; the felspar- crystals, instead of presenting 

 their usual appearance, are dull and full of minute cracks, and appear 

 as if about to fall into small fragments similar to those found in the 

 basins ; the action of the water is evident to the eye though not 

 easily described. An unbroken face of granite resists the weather 

 more powerfully than the rock does when it is broken or penetrated ; 

 in those cases the water soaks into the granite, and thus renders it 

 more easily acted upon by the alternations of heat and cold, wet and 

 dryness. Such action, when once commenced, will continue until 

 checked by the unbroken face of a parting which will limit the ex- 

 tension either perpendicularly or horizontally. The tabular forma- 

 tion of the granite is probably the cause of the frequent occurrence 

 of the basins with flat bottoms. The gradual decay thus acting from 

 a centre will cause the nearly circular and oval forms that so many 

 of the basins present, — the variation from that shape being probably 

 caused by a difference in the structure of the granite. The eye will 

 in a short period discriminate between the Tors where basins would 



