82 RELICS OF PRIMEVAL LIFE 



seated rocks these minerals are formed together, 

 and having crystallized out separately give a 

 spotted and granular character to the mass. 

 Naturally colourless, all these minerals, and es- 

 pecially the felspar and hornblende, are liable to 

 be coloured with different oxides of iron, the felspar 

 usually taking a reddish, and the hornblende a 

 greenish or blackish hue. Now, if we examine a 

 fragment of the oldest or fundamental gneiss or 

 granite, we shall see glassy grains of quartz, reddish 

 or white flat-surfaced crystals of felspar, and dark- 

 coloured prisms of hornblende. When destitute of 

 any arrangement in layers, the rock is granite ; 

 when arranged more or less in flakes or laminae, 

 it is gneiss, the structure of which may arise either 

 from its having been formed in successive beds, or 

 from its having been flattened or drawn out by 

 pressure. These structures can be seen more or 

 less distinctly in any ordinary coarse-grained 

 granite, or with the lens or microscope in finer 

 varieties. 



The Lower Laurentian rocks of our section con- 

 sist essentially of the materials above described, 

 with a vast variety in the proportions and arrange- 

 ments of the constituent minerals. There is, there- 



