524 A TREATISE ON METAMORPHISM. 



impurities. Whitney estimated that to make 1 meter of residual clay in 

 Wisconsin requires 35 to 40 meters of limestone and shale." This involves 

 a diminution of volume of more than 95 per cent. The shrinkage of lime- 

 stone is scarcely less strikingly shown by underground solution, as a 

 result of which the surface becomes uneven and pitted and frequently 

 honeycombed by caves. The process may go so far as to result in the 

 irregular sinking of the surface. This condition is especially well illus- 

 trated by the limestone of Kentucky and by the honeycombed limestones 

 of Lake Huron, described by Bell. 6 



In line with the above is the explanation by Rutley of the slight 

 thickness and lenticular character of many beds of Paleozoic limestone. He 

 holds that the underground waters have dissolved much the greater portion 

 of the limestones, and that these slight layers or lenses are but remnants of 

 the original formations. 



From this maximum diminution of volume represented by limestone 

 there are of course all gradations to no diminution, and doubtless in some 

 cases there is actual expansion of volume. If expansion of volume occurs it 

 would be likely to be in rocks where the minerals are mainly silicates. 

 From the figures given by Merrill and by Watson, it appears that the losses 

 of the original elements in acid, intermediate, and basic igneous rocks vary 

 from 14 to nearly 80 per cent. These losses, as already pointed out, would 

 be partly compensated for by the addition of oxygen and water. More- 

 over, on account of the lower specific gravity of the secondary minerals 

 and the looser state of aggregation, there would be considerable expansion 

 of volume. Hence it appears probable that the decrease in volume in the 

 case of the weathering of these igneous rocks commonly does not exceed 

 50 per cent, and from this the decrease may run down to zero, or even 

 expansion of volume may take place, especially in early stages of alteration. 



EMPHASIS AND RETENTION OF STRUCTURES AND TEXTURES. 



The first effect of weathering upon structures and textures is to 

 emphasize them so that they may be readily seen. Structures which may 

 not be visible in fresh rocks may be strongly marked upon the weathered 



«Hall, James, and Whitney, J. D., Geology of Wisconsin, vol. 1, 1862, pp. 121-125. 



6 Bell, Robert, Honeycombed limestones in Lake Huron: Bull. Geol. Soc. America, vol. 6, 

 1895, pp. 297-304. 



c Rutley, F., The dwindling and disappearance of limestones: Quart. Jour. Geol. Soc, August, 

 1893, pp. 380-381. 



