C.—GEOLOGY. 67 
Peculiarities in the Arrangement of the Minerals in Igneous Rocks.’ He 
showed that the crystals and bubbles in the fluid cavities in granite prove 
its deep-seated origin, and that the relations of augite and leucite in 
Vesuvian lavas demonstrate that the sequence of minerals in igneous 
rocks is determined not by their fusibility but by their order of crystallisa- 
tion out of a cooling solution. 
The determination of the depth and temperature of consolidation of 
an igneous rock by its fluid cavities is not quite so simple and certain as 
Sorby thought; but he explained the anomalies that had led to the 
Durocher-Scheerer controversy, and his method judiciously applied gave 
more positive information than had been previously available. 
The interpretation of mountain structure had an important reaction 
on stratigraphy. Suess realised that the great mountain regions of the 
world were not all due to the folding of the crust; and while vast areas 
have sunk to form the ocean basins, huge horizontal blocks and sheets of 
marine deposits have been left as high plateaux, owing to the subsidence 
of the adjacent areas. 
Suess undertook the study of the world geology to interpret the major 
movements of the crust. He recognised that some encroachments of the 
sea upon the land were world-wide; he called them the marine trans- 
gressions, and explained them by the reduction in size of the ocean basins 
by the uprise of their floors. Lyell had recognised that this movement 
had probably caused some invasions of the land by the sea. Dana 
repeated this view (Manual, 1871, p. 723). Lyell could only regard this 
process as a probability, but Suess had convincing evidence of these 
transgressions from areas which geologically were unknown in the time of 
Lyell. 
Many cases of the rise of the sea surface may be due to changes in the 
oceans’ basins and not to a vertical uplift of the land. Suess went 
further and regarded some high-level horizontal beds as left in their 
original position by the down-sagging of the crust elsewhere during the 
contraction of the earth. Dana had previously (Manual, 1871, p. 723) 
explained the main continental plains at the average height of 1,000 feet 
above sea-level as relics left upraised by the deepening of the ocean 
basins. 
Suess was so impressed by the predominance of downward movements 
in sunklands, rift-valleys, and oceanic deeps, and by the absence of any 
mechanism that he regarded as adequate for widespread horizontal uplifts, 
that he considered all vertical regional movements must be downward. 
Suess went too far in his denial of the possibility of widespread vertical 
uplifts. Various agencies, such as the subcrustal flow of material displaced 
by an oceanic subsidence, will uplift areas without appreciable tilting. 
But that the land sometimes emerges owing to lowering of the sea surface 
_ and at others is submerged by the rise of the sea-level is now universally 
admitted. Suess’ transgressions give geology a physical basis of cor- 
relation more precise and world-wide than is possible from paleontology. 
_ A transgression may be simultaneous over the whole world, whereas any 
special fauna does not appear at all places at precisely the same time. 
This fact was early recognised by many geologists. 
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