VARIABLE AMOUNT OF WEATHERED MATERIAL. 531 



to a depth of 60 meters." The Transvaal granites, according to Furlonge, 6 

 are decomposed to a depth of 60 meters. 



From maximum amounts of weathered material illustrated by the 

 above regions there are all gradations to amounts almost ina23preeiable. 

 Excellent illustrations of such regions are the areas of the Northern 

 Hemisphere over which the latest ice incursion has taken place. Glaciated 

 quartzites may retain the glacial striations with marvelous delicac}", micro- 

 scopic strise being perfectly preserved. Such igneous rocks as the granites, 

 gabbros, and peridotites, and other families usually show only a mere skin 

 of appreciably decomposed material, ordinarily but a fraction of an inch 

 in thickness. The readily soluble limestones usuall} T present planed, 

 grooved, and striated surfaces which show comparatively little evidence of 

 modification. The minimum weathering effects are found on the solid 

 rocks which have a thin veneer of drift or soil. Where this veneer is thick 

 enough to bury the rock surface below the frost line, the only weathering 

 effects ordinarily produced are slight stains of decomposition on the surface 

 and along the joints and other fractures. Where the topographic forms 

 are more rugged, so that the rocks are more exposed to weathering, there 

 has been pronounced disintegration, and the decomposition has also gone 

 farther. While the latest drift shows much more evidence of weathering 

 than the solid rocks, below the soil the weathering effects are surprisingly 

 slight. The pebbles and bowlders commonly show planing, grooving, 

 and strise, and scarcely any evidence of decomposition. The marvelous 

 freshness of this drift, which has been exposed to the weathering forces 

 under favorable circumstances for thousands of years, perhaps tens of 

 thousands of years, is to me ,the best evidence of the slowness of the 

 complex process of weathering. 



A comparison of the small amount of weathering in the glaciated 

 regions with the great amount in some other regions gives one an idea of 

 the vast amount of time required for deep and advanced weathering. 

 Even if the most conservative estimate as to the length of time since the 

 last ice invasion were used as a measure of the rate of weathering, it would 

 follow that to have accomplished such weathering results as are exhibited 



« Belt, Thomas, The naturalist in Nicaragua, p. 86. 



!>Furlonge, W. H., Notes on the geology of the Dekaap gold fields in the Transvaal: Trans. Am. 

 Inst. Mm. Eng., vol. 18, 1890, p. 337. 



