WEATHEEING INFLUENCED BY COMPOSITION 235 



cies more readily than the potash varieties, basic eruptives do 

 not in all cases decompose more rapidly than the granitic rocks 

 into which they are intruded, as is well illustrated in some of the 

 glaciated areas about Boston, where small, compact dikes form 

 low ridges a few inches above the surface of the enclosing granite. 

 Much depends upon the grain of the rock and the character of 

 the secondary minerals which have been generated at some 

 period prior to its decomposition proper. <Thus those dikes con- 

 taining so large a proportion of secondary epidote as to be of a 

 dull greenish hue are almost invariably more enduring than the 

 granites, while those on the other hand, in which the secondary 

 minerals are largely chlorite, calcite, and zeolitic compounds, 

 yield to the decomposing agencies more readily. Even when the 

 dike as a whole gives way, the presence of epidotie aggregates 

 frequently manifests itself in protruding knots and bunches 

 above the corroded surface. Knots caused by segregations of 

 black tourmalines stand out in the same way from the surface 

 of Stone Mountain, already referred to. Garnets, staurolites, 

 quartz veins, and other of the less easily decomposed minerals 

 may stand out in like manner from the surface of the rocks 

 of which they form a part. 



Granitic and other complex crystalline granular rocks will, on 

 exposure, sometimes take on a pitted surface, owing to the re- 

 moval of the more easily decomposed materials. The boulders 

 of nepheline syenite in the glacial drift about Portland, Maine, 

 are thus corroded to the depth of several millimetres through 

 the removal of the granular nepheline, while the feldspars and 

 hornblendes project irregularly. 



Calcareous rocks containing silicates, like the amphiboles or 

 pyroxenes, show like roughened surfaces due to the dissolving 

 away of the calcareous matter, leaving the silicates projecting 

 (Fig, 2, PL 14), or, as is the case with some of the tremolite- 

 bearing dolomites used for building, may become pitted by the 

 dropping out of the tremolite as the calcareous cement gives way.^ 



Many sandstones become likewise roughened through the re- 

 moval of a portion of the cementing constituent, leaving the 

 siliceous granules projecting. In the coarsely crystalline lime- 

 stones and dolomites the solution and weathering effects are 

 often first manifested along cleavage lines and the contacts of 



^As in the XJ. S. Capitol Building at Washington, 



