GEOLOGY 
quantities of fine dust constantly suspended in the air, than by the tender 
blue of the purer skies with which we are familiar.’ 
Perhaps it may be as well to mention in this place, that concurrently 
with the progress of these events in the northern parts of the kingdom, 
geographical conditions of a different kind existed further south and south- 
east. In the areas referred to, marine sediments, including important beds 
of marine limestone, were in process of formation. These are well seen 
in the Rhineland, and almost equally well in Devonshire, where they 
were first studied by geologists. For the latter reason the southern type 
of rocks is termed the Devonian Rocks, and the period when they were 
formed, the Devonian Period. Henceforth, therefore, the events now 
under description will be referred to here as having occurred during 
the Devonian Period. But the northern type of rocks, which consist 
largely of sandstones of a dominant red colour, will still be referred to as 
the Old Red Sandstone Rocks. 
(4) The Devonian Period in the northern parts of the kingdom was, 
as already mentioned, one of considerable terrestrial disturbances, which 
manifested themselves by great local upheavals, accompanied by earth- 
quakes, and followed by volcanic outbursts, which eventually assumed 
extensive proportions. With the volcanoes themselves, as well as with 
the stratified rocks that grew up with them, we happen not to be very 
much concerned, for reasons which will be stated presently. But the 
former presence of the volcanoes has left its mark in Cumberland in a 
striking manner, and in many different ways, the nature of which will 
be considered after the following preliminary explanation. 
(c) There is reason to believe that within the lower part of the core 
of a volcano the rocks have been reduced to a pasty or semifluid condition 
by the uprise of those superheated alkaline waters, which have already 
been mentioned as forming one of the principal factors in all volcanic 
eruptions. It is within this plutonic region, which may be situated 
several miles below the summit of the volcano, that such rocks as granites 
and the rocks allied thereto are generated. Indeed, there is reason to 
believe that the greater part of all such plutonic masses have originated 
deep within the earth’s crust at the root of a volcano. In other words, 
areas of granitic rocks generally mark the site of former volcanoes. The 
zone within which these rocks are generated may conveniently be re- 
ferred to as the ‘ granitic zone.’ 
Furthermore, the same superheated alkaline waters, whose uprise is 
so essentially connected with volcanic action, permeate the sedimentary 
and other rocks contiguous to the lower part of a volcano, and there pro- 
duce very important changes by giving rise to what is termed contact 
metamorphism. This zone may be referred to as the ‘zone of thermo- 
metamorphism.’ 
If the reader will bear these general principles in mind it will 
enable him to understand the nature and origin of some important 
1 Goodchild, ‘Desert Conditions in Britain,’ Trans. Geol. Soc. Edin., vii. pp. 203-222 
(1896-97). 
I 17 c 
