116 Pirsson—Complementary Rocks and Radial Dikes. 
Art. XIII.—Complementary Rocks and Radial Dikes ; 
by L. V. Pirsson. 
THE progress of petrological research within the past two 
decades has shown that areas of intrusion of massive igneous 
rocks are very frequently accompanied by smaller, less promi- 
nent intrusions, usually in the form of dikes and sheets, of 
rocks which in part are more acid and in part more basic than 
the main bodies with which they are geologically connected. 
The study of the various types of rocks occurring at erup- 
tive centers has shown that they are caused by the differentia- 
tion of igneous magmas, and that this differentiation in the 
main is due to the local concentration of lime, iron, and magne- 
sia, by processes not yet fully understood. By this means arise 
bodies of magma, some relatively richer in silica, alumina and 
alkalies, others richer in lime, iron and magnesia than the 
original fluid from which they were formed. That such differ- 
entiation does take place seems now well established—in late 
years alarge amount of indirect proof of it has been given 
and quite recently cases have been presented which furnish 
actual demonstrations of the fact.* 
These smaller rock bodies then, if differentiation is admitted, 
are the resultants of a longer and more complete phase of the 
process than that represented by the main masses which they 
accompany, and it follows that they are younger in point of 
age. 
“The recognition of the genetic connection between these 
smaller rock bodies and, the masses they are found with we 
owe chiefly to Rosenbusch, who perceived the association 
between the dikes of minette and vogesite and the granitic 
areas they accompany in eastern Germany. osenbusch, how- 
ever, extended this idea of relationship still further and insisted 
that these smaller rock masses had their appearance strictly 
conditioned, that they typically appeared only in the form of 
dikes and accordingly as the main bodies or stocks which they 
accompanied were composed of rocks of different types, so also 
the dikes varied in type and the correspondence was absolute. 
That is that each kind of stock rock had its own particular 
satellites in the form of special dike rocks. This is, of course, 
well known to all petrographers. 
Iddings,t however, has shown that these smaller, more highly 
differentiated bodies of magma, at eruptive centers, may appear 
* Harker, Quar. Jour. Geol. Soc., vol. ], p. 311, 1894. Weed and Pirsson, Bull. 
Geol. Soc. Amer., vol. vi, pp. 389-422, 1895. 
+ Origin Igneous Rocks, Bull. Phil. Soc. Washington, vol. xii, p. 167, 1892. 
