220 EIGHTEENTH CENTURY. PT. in. 



Igneous (or heat-made) Hocks. But how are we to 

 decide about those rocks, such as basalt, which Werner 

 thought were made by water? Hutton was convinced they 

 were formed in volcanoes ; and yet it was true that they did 

 not contain bubbles of air as lava does, which has poured 

 down the sides of a volcano in the open air. Here his 

 friend and pupil Sir James Hall came to his assistance by 

 melting pieces of rock in his chemical laboratory, and letting 

 them cool down under very heavy pressure. When this was 

 done they could hardly be distinguished from pieces of 

 basalt which he took out of the earth. It is clear, there- 

 fore, he said, that these rocks have either cooled down in- 

 side the volcano, with a great weight of rocks above them, 

 or have been poured out under the sea, which would press 

 down heavily upon them and shut out the air. 



Another question which Hutton cleared up in the same 

 way was that of the formation of granite. Werner believed 

 that all the granite rocks, of which you may see plenty in 

 different parts of the world, were made first, before any other 

 rocks were laid down by water. Hutton did not think this 

 was true, but that, on the contrary, granite might be even 

 now forming deep down within the crust of the earth. But 

 how was he to prove this ? He said to himself, ' If melted 

 granite forms under the softer strata which have been laid 

 down by water, it ought occasionally to obtrude itself into 

 them in narrow wedges when it is expanded by heat, and I 

 shall be able somewhere to find veins of granite piercing the 

 rocks above.' 



To prove whether this was so he made a journey to the 

 Grampians, where there are large masses of granite ; and 

 there, in Glen Tilt, he found the veins of red granite branch- 

 ing out into the clay-slate and limestone rocks above, as 

 in Fig. 3g. It is easy in this diagram to see that the 



