262 TIME CONSIDBBATIOjNTS 



more superficial portions being reduced to the condition of a 

 red clay, while the lower are merely rendered soft and friable, 

 with little if any change in color. This disintegration has gone 

 on to such an extent that where the rock is traversed, as is 

 sometimes the ease, by numerous gold-bearing quartz veins, the 

 entire mass of material is washed down by water — hydraulicked 

 — as in the ordinary process of placer mining. The Pliocene 

 andesites are also in places decomposed to a depth of 20 feet. 

 The region is one of heavy annual precipitation, but the rain- 

 fall is limited almost wholly to the winter season. 



Eoek disintegration and decomposition, after the manner 

 already described, have been by no means limited to the present 

 era, but have been going on since the first land appeared above 

 the surface of the primeval ocean. The results of the recent 

 decomposition are more apparent, since the derived materials are 

 still recognizable as rock debris, while that formed in past ages 

 may have been so changed by the solvent and assorting power 

 of water, the chemical action of the atmosphere, and the general 

 agents of metamorphism, as to have quite lost its identity. 



Dr. R. Bell, of the Canadian Geological Survey, has described^ 

 an interesting illustration of pre-Pala302oic decay in the crystal- 

 line rocks north of Lake Huron. The red granite, where it has 

 been protected from glacial action, is found to be eaten into 

 hollows in the form of round and sack-like pits and small 

 caverns, the last-named generally occurring on steep slopes or 

 perpendicular faces of the rock. These pits are most usually on 

 sloping surfaces, and in places are of sufficient size to allow two 

 men to crouch within. The granite around these pits shows no 

 indications of decay. That they are of pre-Palseozoic origin is 

 demonstrated by the presence in them of residual patches of the 

 fossiliferous Black RiVer limestone, which Professor Bell regards 

 as veritable inliers of the Black River formation once filling all 

 the inequalities and still overlying the granite at lower levels, 

 though elsewhere almost wholly removed by erosion. Figure 23, 

 after Bell, shows diagrammatically the old granitic corroded 

 &00T upon which the calcareous sediments were laid down, with 

 pits still containing residual masses of the limestone, and the 

 still intact beds passing under the waters of Lake Huron at the 

 lower right. 



^BiiH. Geol Soc. of America, Tol. Y, 1894, pp. 35-37. 



