48 



In determining specific gravity, it must be remembered that different 

 portions of one and the same rock-mass will often yield different results. 

 In some cases the differences are slight ; in others they are considerable. 

 As a general rule the range of variability appears to be greater in plutonic 

 than in volcanic masses. The Cleveland, Cockfield and Armathwaite dyke 

 may be quoted as an illustration of an igneous mass which exhibits a 

 somewhat striking uniformity in specific gravity over great distances. 

 This dyke can be traced at intervals from Maybecks, near "VVhitby, to 

 Armathwaite in the Eden Valley: a distance of 90 miles. The specific 

 gravities of many specimens of the unaltered rock taken from widely- 

 separated localities were found to lie between 2'765 and 2'788. No other 

 dyke in the North of England was found to possess an average specific 

 gravity lying between these limits. As might naturally be expected, a 

 constancy in the specific gravity of any igneous mass goes along with a 

 constancy in the other chemical and physical properties. 



The relation between the specific gravity of a rock and the specific 

 gravities of the constituents of which it is composed may be expressed as 

 follows : Let A, B, c, &c., represent the proportions by weight of the 

 individual constituents, so that A + B + c, &c., = 100, and let a, b, c, &c., 

 represent the specific gravities of the individual constituents. Then the 



specific gravity of the rock as a whole will be ~ + + ~&c. In the 

 case of a rock composed only of two constituents, having different specific 

 gravities, the relative proportions of the two constituents may be roughly 

 determined when the specific gravity of the rock as a whole is known, and 

 that of each of the constituents. 



The principal interest which attaches to specific gravity, so far as 

 igneous rocks are concerned, lies in the fact that it stands in close relation 

 to the other properties, both chemical and physical. If we compare rocks 

 in the same physical condition, then the specific gravity is seen to vary 

 with the chemical composition in a tolerably definite manner. If, on the 

 other hand, we compare rocks of the same composition, then the specific 

 gravity is seen to depend on the physical condition. In the previous chapter 

 it has been shown that the variations in the chemical composition of igneous 

 rocks are expressed in a broad and general way by BUNSEN'S law ; so that the 

 percentage of any one of the variable constituents gives a certain amount of 

 information with regard to the relative proportions of the remaining 

 constituents. Of the variable constituents silica is unquestionably the one 

 which throws the greatest amount of light on the composition of the rock 

 as a whole. The following table illustrates the general laws with regard to 

 the variations in the specific gravities of the different kinds of igneous rocks. 



