40 A NATURALIST IN THE GREAT LAKES REGION 



great cracks and crevices fresh lavas forced their way, often 

 separating the strata, raising the upper ones as domes and filling 

 the spaces so formed. So these rocks where they are accessible 

 today are seamed with dykes (Fig. 25), as these cooled lava 

 intrusives are termed, and where their upper layers have been 

 removed by erosion the old masses of lava that filled the domes 

 stand up as rounded hills or monadnocks (Fig. 26). 



How often this process of rock destruction, reformation, and 

 metamorphosis was repeated we do not know. In time simple 

 plants and animals appeared. Through their action beds of 



FIG. 26. A monadnock, Ishpeming, Michigan 



carbonaceous material probably peat of iron oxide, and of 

 magnesian limestones were laid down with the sandstones and 

 shales. These also were transformed by metamorphosis into 

 graphites, iron ores, and dolomitic marbles. The early lavas^ 

 granites, diorites, and such were in many cases also greatly 

 altered by later strains and stresses, to which they were sub- 

 jected by crustal movements. So the earliest rock masses to 

 which we have access consist of highly nietamorphic quartzites, 

 slates, schists, marbles, seamed and infiltrated with granites and 

 with basic lavas that are often so abundant as to dominate the 

 formation. These old rocks are so folded, crumpled (Fig. 27), 



