76 A NATURALIST IN THE GREAT LAKES R. - 



is present and also often mica and hornblende but not in predomi- 

 nating quantities. The diorites are plutonic, the corresponding 

 volcanic rocks being the andesites. 



Gabbro and basalt are respectively the plutonic and the 

 volcanic members of the next family. They are dark, heavy 

 rocks with labradorite and augite as the predominant minerals. 

 Biotite, chlorite, magnetite, pyrite, olivine, etc., may be accessory 

 minerals. The gabbro is distinguished from the diabase by its 

 coarser crystallization and the large quantity of pyroxene it 

 contains. The presence of chlorite in these rocks gives them a 

 distinct green color and they are then known as greenstones. It 

 is difficult to distinguish the finely crystalline ingredients of 

 trachyte and andesite in the field sufficiently to distinguish 

 them, so they are usually called collectively "felsite" as dis- 

 tinguished from the darker and heavier basalt. 



Metamorphic rocks. Both sedimentary and igneous rocks 

 may be altered by pressure, crumpling, heat, and other agencies, 

 so that their original character is quite changed; such rocks are 

 called metamorphic rocks. Thus the limestones become crystal- 

 line and transform to marbles, as do also the dolomites. The 

 effervescence with acid still distinguishes them. Dolomitic 

 marbles effervesce only in strong or hot acid, the ordinary marble 

 in cold, even weak, acid. Sandstone is compacted, the sand 

 grains fused to make quartzite a rock that may have any color 

 but can be distinguished by its hardness and conchoidal fracture. 

 It is not as vitreous in luster as is quartz itself, and shows more 

 or less its granular character. Shales are changed to slates, 

 recognized by their easy separation into thin layers like roofing 

 slate. Sedimentary rocks composed of the disintegration of 

 granites are by metamorphosis altered to schists and gneiss. 

 The latter contains the same minerals as granite, but there is 

 evidence of stratification and the crystals or grains of the com- 

 pressed minerals are arranged with their long axes in one plane. 

 If the compression has been so great as to flatten the component 

 grains or crystals into scales, the rock becomes a schist, easily 



