Metamorphism. 47 



have been changed in character for a short distance 

 from the agent that has been employed in effecting 

 that minor kind of metamorphism (figs. 4 and 9). 



On a much larger scale, the phenomena we meet 

 with in a truly metamorphic region are as follows. In 

 the midst of a tract of mica-schist, gneiss, or other 

 altered rocks, a boss of granite (or one of its allies) 

 rises, like those for instance of Dartmoor and Cornwall 

 or of the north end of the Island of Arran. At a dis- 

 tance from the granite the beds may consist, perhaps, 

 of unaltered shale, or of slate, sandstone, and limestone. 

 As we approach the granite, the limestones become 

 crystalline, and often lose all traces of their fossils ; 

 the sandstones harden and pass into quartz-rocks, and 

 the shales or slates, or sandy beds and shales, lose their 

 ordinary bedded texture, and pass by degrees into mica- 

 schist, or perhaps gneiss, in which we find rudely 

 alternating laminae of quartz, felspar, and mica, often 

 arranged in gnarled or wavy lines (foliation, figs. 1 and 

 11). As we approach the granite still more closely, we 

 find possibly that, in addition to the layers of mica, 

 quartz, and felspar, distinct crystals^ such as garnets, 

 staurolites, schorl, &c., are developed near the points 

 of contact, both in the gneissic rock and in the granite 

 itself. 



It is not necessary for my argument that I should 

 describe these minerals. It is sufficient at present to 

 state the fact that such minerals are developed under 

 these circumstances, and this is due to the influence of 

 metamorphism. 



Furthermore in some cases, as in the Laurentian 

 rocks of Canada, great thicknesses of interstratifted 

 gneiss are so crystalline that, when a hand specimen 

 or even a small part of the country is examined, they 



