Pirsson— Complementary Rocks and Radial Dikes. 117 
in extrusive forms as well as intrusive ones, but that the min- 
eral composition in each case depends largely on the physical 
conditions under which the magma cooled and crystallized. 
And a priori it does not seem reasonable to expect that such 
bodies of magma should appear only in the form of dikes but 
in intrusive sheets, laccolites, and smaller masses as well, while 
the smaller more highly differentiated magma of one locality 
may be the common type or main one at some other center. 
Brogger in his interesting article on the basic rocks of Gran* 
has shown moreover that the types which these accompanying 
rocks assume are not necessarily conditioned by the type of the 
main bodies and that in the district studied by him camptonites 
and bostonites occur as the differentiation-products of a gabbro 
magma, and are not therefore attached unconditionally to neph- 
elite syenite, as Rosenbusch had previously assumed. 
Bearing the above facts in mind, it is nevertheless certainly 
true that the appearance of these companion rocks at centers of 
igneous intrusion is, as stated at the commencement of this 
article, a very frequent, though not anecessary one. And in 
those localities where the deep-seated rocks have been laid bare 
by erosive dissection, they will most commonly appear as dikes 
and intrusive sheets. It seems to us that to lose sight of, or to 
deny this, is to neglect one of the most useful and important 
features of general petrology. 
For these accompanying rocks Broggert has proposed the 
very apt term of “complementary rocks,’ meaning thereby 
that the basic forms (the high iron, lime, magnesia, low silica 
type) if combined in correct proportion with the acid ones (the 
high silica, alumina and alkalies) would give the composition of 
the main type of magma which they accompany and from 
which they have arisen by differentiation. Thus they comple- 
ment each other. 
To the basic forms of complementary rocks,—the types rich 
in ferro-magnesian minerals, occurring in this manner,—the 
general term of “lamprophyre”’ has long been employed. It 
is true that some object to the use of the term in this sense and 
the objections have been well stated by Zirkel in the recent 
edition of his Lehrbuch der Petrographie.t The term is, how- 
ever, insuch common use by the majority of petrographers 
that it would be probably impossible now to displace it. It 
makes less difference what the term is than that we have it and 
use it with a common meaning. The objections to it are based 
almost wholly on etymological grounds and we might as well 
object to the use of “ porphyry ” in a general sense because the 
majority of the rocks to which it is now applied are not red. 
* Quar. Jour. Geol. Soc., vol. 1, p. 15, 1894. 
+ Loc. cit. 
t{ Zweite Auflage, vol. ii, p. 341, 1894. 
