178 
IDDINGS. 
igneous rocks, if it is to be based on genetic relationships, 
cannot be founded on the geological mode of occurrence of 
such rocks, either as surface flows, or dikes and intrusive 
sheets, or laccolites and batholites, or as deeply-seated or 
superficial masses. 
LOCALIZATION OF VOLCANIC PHENOMENA. 
Nothing is more striking upon studying a great region of 
volcanoes or of volcanic rocks than the local character of vol¬ 
canic phenomena. It has been specially commented on by 
King in the discussion of the origin of volcanic species, 
already cited in the historical part of this paper. Active vol¬ 
canoes within the same district not only erupt independently 
of one another, but produce lavas of widely different com¬ 
position. They are thus immediately connected with differ¬ 
ent kinds of magmas. A region of extinct and dissected 
volcanoes, such as those on the outskirts of the Yellowstone 
Park, furnishes a still better idea of the localized character 
of volcanic activity, because the complete series of events 
which produced at each center a great variety of rocks can 
be observed, instead of the external phenomena at a partic¬ 
ular period in the history of a group of active volcanoes. 
Irregular order of succession .—The investigation of such a 
region shows that at each center of volcanic action or at each 
volcano there has been erupted a series of different kinds of 
rocks, which followed one another in a definite order, with 
slight exceptions; and, further, that at various centers the 
range of the series of rocks has been different in extent and 
also different in kind; one group being mainly basic magmas, 
another mostly acidic, and so on. Moreover, the series at 
the various centers may overlap one another in the range of 
their variations, producing a recurrence of varieties ; and 
since neighboring centers may be erupting different phases 
of the rock series at one and the same time, as w x e learn from 
active volcanoes, where the ejectamenta of different volcanoes 
commingle, the same kinds of rock may occur in different 
parts of the whole complex series representing the order of 
