GEOLOGY OF THE INNER EARTH—GREGORY. 321 
periods of quiescence. The geographical effects of changes in the 
earth’s quivering mass affect distant regions at the same time. There- 
fore the landmarks of physical geology will probably be found to give 
more precise evidence as to geological synchronism than those of 
paleontology, on which we have hitherto had to rely. 
PLUTONISTS AND ORE FORMATION. 
Belief in the earth’s internal fires was most faithfully held 
amongst geologists by the Plutonists of the eighteenth century 
and repudiated with equal thoroughness by the Neptunists, who 
refused to concede that volcanic action was due to deep-seated 
cosmic causes. Thus Jameson in 1807 stoutly maintained that vol- 
canoes were superficial phenomena due to the combustion of beds 
of coal beneath fusible rocks, such as basalt, and that the explo- 
sions were due to the sudden expansion of sea water into steam 
by contact with the burning coal. Volcanoes, according to this 
view, were correctly described as burning mountains, giving forth 
fire, flame, and smoke. The extreme Neptunist and Plutonist 
schools have long since been extinct, but the controversy is not quite 
closed. The battlefield is now practically restricted to economic 
geology, and the issue is the origin of some important ores. 
Ore deposits present so many perplexing features that deep-seated 
igneous agencies were naturally invoked to explain them, and some of 
the most thoroughgoing champions of the igneous origin of ores 
make claims that remind us of the eighteenth-century Plutonists. 
The question is to some extent a matter of terms. Many of the ores 
which Vogt, for example, describes as of igneous origin he attributes, 
not to the direct consolidation of material from a molten state, but to 
eruptive after actions due to the hot solutions and heated gases given 
off from cooling igneous rocks. Igneovs rocks probably play a 
notable part in the genesis of most primary ore deposits; for the 
entrance of the hot ore-bearing solutions is rendered possible by the 
heat of the igneous intrusions, as Professor Kemp has well shown in 
his paper on “ The Role of Igneous Rocks in the Formation of Metal- 
he Veins.” Professor Kemp limits the term “igneous” to materials 
formed by the direct consolidation of molten material; and this de- 
cision seems to me to be most convenient. For example, the quartzite 
that is so often found beneath a bed of basalt is due to hot alkaline 
water from the lava cementing the loose grains of sand; the process 
is an eruptive after action, but it would be unusual to call such a 
quartzite an igneous rock. 
1. [gneous ores—That there are ores which are the products of 
direct igneous origin is now almost universally admitted. The 
mineral magnetite is a most valuable source of iron, and it is a 
