cii. XXVI. IGNEOUS ROCKS. 



everywhere signs that these gradual and incessant changes 

 have ahvays been going on, and that the face of our earth, 

 as we now see it, has been moulded out of the ruins of an 

 older world. 



Igneous (or fire-made Rocks). — But how are we to decide 

 about those rocks, such as basalt, which Werner thougln 

 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 laborator}'^, 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 dif- 

 ferent parts of he 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 form- 

 ing 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.' 



