THE HARDENING OF ROCKS 



rock have been thrust up through a bed of coal, the 

 coal is changed or " metamorphosed " where the basalt has 

 pierced it. Sometimes it becomes hard coal like anthracite ; 

 sometimes it is changed into graphite the black rock 

 of which pencils are frequently made. 



Limestone pierced by basalt becomes marble. When 

 sandstone is discovered in contact with ancient volcanic 

 rock it is found to have lost its reddish colour, and to 

 have become white, grey, green, or black. It separates 

 into crystals; it becomes glassy and hard. All these 

 instances are those of rocks which we can perceive to 

 have been altered by coming into contact with great heat. 

 But there is another kind of change of a very much more 

 widespread character which can be perceived among the 

 most ancient of those rocks which we know must have 

 been first quietly laid down as sediments. It is sometimes 

 spoken of as " general metamorphism. 1 " 



This widespreading change may extend over great 

 regions and vast extents of country. The most striking 

 series of such rocks was first described by Sir W. E. 

 Logan, Director of the Canadian Geological Survey ; and 

 he estimated the thickness of them at 30,000 feet. They 

 lie beneath all the unaltered rocks, and are (in North 

 America) the rocks which were the base or foundation 

 of the North American continent before the later sedi- 

 mentary rocks were laid down on them. They are called 

 the Laurentian rocks, because they were first found in the 

 neighbourhood of the St. Lawrence River ; but they exist 

 in many places besides Canada and North America ; and 

 the foundations of Scandinavia and of the Hebrides are 



