THE ORIGIN OP IGNEOUS ROCKS. 
129 
the mineral composition of the various bodies of rock, even 
when the more thorough study brings to light the great 
number of intermediate varieties. In fact, when large areas 
of eruptive rocks are carefully investigated, it is found that 
there is a perfect and gradual transition of one kind into 
another—all intermediate varieties existing—and that quan¬ 
titatively no special part of the series is universally pre¬ 
dominant, although there are often immense masses of 
nearly uniform character, and there may be smaller bodies 
of quite variable composition. Nevertheless, the effort is 
made in describing such a group of rocks to select the rocks 
that differ most and make them serve as types to which 
specific names can be applied, alluding to the others inci¬ 
dentally as intermediate forms. The convenience as well 
as the error of such a method are apparent. 
When, however, the rocks of many regions are studied, 
it is found that, notwithstanding the variations in mineral 
composition in the rocks of one district, there exist charac¬ 
teristic qualities of the minerals which are persistent for a 
greater or less range of rock varieties in that district, and 
which distinguish the whole group of rocks from those of 
some other district. The qualities or peculiarites are like 
family traits of character, and at once suggest the intimate 
relationship and common origin of all of the igneous rocks 
of the district. The same idea has been clearly expressed 
by Judd,* who states that “there are distinct petrographi- 
cal provinces within which the rocks erupted during any 
particular geological period present certain well-marked 
peculiarities in mineralogical composition and microscopical 
structure, serving at once to distinguish them from the 
rocks belonging to the same general group, which were 
simultaneously erupted in other petrographical provinces.” 
It is, of course, not to be inferred from this that every dis¬ 
trict of eruptive rocks has some peculiar characteristic which 
*Judd (J. W.) “On the Gabbros, Dolerites, and Basalts of Tertiary- 
age in Scotland and Ireland.” Quart. Journ. Geol. Soc. 8°. London, 
1886, vol. 42, p. 54. 
20-Bull. Phil. Soc., Wash., Vol. 12. 
