Igneous Rocks. 19 



among the Silurian rocks of the mainland of Scotland. 

 Even the large masses of granite there, occupy but 

 small areas when compared with the great extent of 

 ordinary stratified and metamorphic rocks amid which 

 they lie. It is chiefly in the Inner Hebrides that great 

 masses of tertiary basalts occur. Igneous rocks exist 

 even in much smaller proportions in Derbyshire, North- 

 umberland, Devon, and Cornwall, excepting the occa- 

 sional occurrence of large -bosses of granite in the two 

 last-named counties, as for example on Dartmoor, and 

 at Land's End. If, however, we examine all the mid- 

 land, southern, and eastern parts of England, we shall 

 find hardly any igneous rocks whatever. 



I have now briefly to indicate how we are able to dis- 

 tinguish igneous from aqueous rocks, in countries where 

 there are neither active nor obvious craters of extinct 

 volcanoes, such as those of Auvergne and the Eifel. 

 To do this in detail would occupy a volume. 



In a general way we can distinguish them from 

 strata formed by aqueous deposition because many of 

 them are unstratified, and have other external and 

 internal structures different from those of aqueous 

 deposits. To take examples : If we examine the lavas 

 that flowed from any existing volcano, and have after- 

 wards consolidated, we find that they are frequently 

 vesicular. This vesicular structure is largely due to 

 watery vapour, and partly to gases ejected along with 

 the melted matter, which, expanding in their efforts 

 to escape from the melted lava, form a number of 

 vesicles, just as yeast does in bread, or as we see in 

 some of the slags of iron furnaces, which, indeed, are 

 simply artificial lavas. This peculiar vesicular structure 

 is never found in the case of unaltered stratified rocks. 

 Here, then, experience tells that modern rocks with this 



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