14 THE CHICAGO ACADEMY OF SCIENCES 



greatest quantity of minerals, while the drift which is com- 

 posed of the materials derived from six geological systems- 

 Devonian, Silurian, Odovician, Cambrian, Algonkian, and 

 Archaean and from a wide extend of country, presents the 

 greatest variety of species. 



This diversity of our minerals is further increased by the 

 changes which the minerals often undergo, whereby secondary 

 forms result from the original. 



