ORIGIN OF IGNEOUS AND METAMORPHIC ROCKS 263 



fine, parallel lines that are present in stratified rock. If now the 

 sediment were to become very thick, producing great pressure on the 

 lowest layers, and if a cementing material were present, the clay de- 

 posited in the pond might be changed to shale, and the sand to sand- 

 stone. 



288. Origin of Igneous and Metamorphic Rocks. We 



cannot observe the formation of igneous rocks as easily 

 as that of stratified rocks; but there are, even now, vol- 

 canic regions in which melted rock is being expelled from 

 the interior of the earth. 

 Sometimes the melted 

 rock hardens to a glassy, 

 brittle rock, forming ob- 

 sidian and lava (Fig. 227) . 

 Sometimes the volcanic 

 material is full of gas 

 bubbles, and hardens to 

 form pumice. 



New England has no 

 volcanic conditions to- 

 day, but there is every 

 evidence that it had them in the past ; for in many places 

 great dikes of igneous rock cross its other rocks, and these 

 are nothing but rock cracks that have been filled, from 

 below, with lava. In Oregon great areas of lava are 

 found on the surface. 



Granite differs from lava and other igneous rocks in that it seems to 

 have been pushed into cracks and pockets entirely beneath the earth's 

 surface. Its minerals must have crystallized out during a long period 

 of very slow cooling. When we now find granite at the earth's surface, 

 we believe that the rock which once covered it has been worn away. 



Fig. 227. 



A Roman Mill; Made of Lava. Field Museum 

 of Natural History. 



