THE PROTEROZOIC ERA. 



207 





Fig. 81.— Fracture cleavage developed by pressure shown in the left half of the figure, 

 while it is wanting in the right half. The vertical line is a bedding plane. Ihe layer 

 to the left was of sufficiently different composition, or subject to sufficiently different 

 movement, to develop fissility, while that to the right was broken (brecciated) 

 instead. The rock at the left would be called quartz schist, while that at the right is 

 quartzite. Animikean formation near Ableman, Wis. (Atvvood.) 



