118 Pirsson—Complementary Rocks and Radial Dikes. 
What we do need, however, in petrographic nomenclature at 
the present time, is a general term for the acid forms of the 
complementary rocks. By some the term “aplitic form” has 
been used but has never become general. The author would 
suggest in this connection that for these acid types the general 
term of oxyphyre be employed. 
These two terms lamprophyre and oxyphyre should then be 
regarded as correlative of each other, and taken together for 
any one district where such relations exist, they will form its 
complementary rocks. And it is to be understood that these 
terms are not to be used in a specific but in a general sense— 
subjectively, not objectively. It may well happen that a type 
of rock which in one district is a lamprophyre or an oxyphyre 
may in another locality be the main predominant type. And 
of course in many places no such complementary relations may 
exist. 
Radial Dikes.—It is a fact well known to most petrologists 
that eruptive centers are frequently surrounded by mazes of 
dikes. Not only are the central stocks or massives cut by these 
dikes but they ramify outward among the tuffs, breccias and lava 
flows, if extrusive action has taken place, or if erosive agencies 
have carried these away, they are found in an outer zone among 
the sedimentaries, often extending for great distances. Insuch 
cases it is often to be noticed that the dikes have a pronounced 
radial disposition with respect to the central stocks with which 
they are connected. Instances of this will probably suggest 
themselves at once to most geologists and a very marked one 
is afforded in the Crazy Mts. in Montana, the radial disposition 
of whose dikes was first observed by Iddings as stated by 
Wolff*. The Livingston sheet of the Geological Atlas of the 
U.S. Geological Survey presents in a most graphic way this 
relation of the dikes in the southern portion of the Crazy Mts. 
Another very remarkable case of these radial dikes is afforded 
by the Highwood Mts. in Montana, an account of which has 
been recently given by W. H. Weed and the author.t 
The origin of these dikes is by most geologists referred to 
the shattering of the strata by the force of the volcanic out- 
breaks which gave rise to the center of eruption and the subse- 
quent filling of the cracks by intruded magmas. The idea 
might be illustrated by the disposition of the hole and cracks 
formed in a pane of glass when a missile has been violently 
hurled through it. 
While by no means denying that dikes at voleanic centers 
may be and are formed in this manner, it seems evident to the 
writer that they cannot all have originated in this way, and this 
* Bull. Geol. Soc. America, vol. iti, p. 451, 1892. 
+ Ibid., vol. vi, pp. 389-422, 1895. 
