IQO Bulletin of Laboratoincs of Dcnison University. LVoi. xii 



The Crystalline area forms the middle belt of the State 

 (Fig. i). It is separated on the southeast from the Coastal 

 Plain by the fall line, and on the northwest it is separated from 

 the Paleozoic area by the Cartersville fault. Its axis has a gen- 

 eral northeast-southwest trend, and, with the exception of the 

 extreme ten northwest counties, it occupies the entire north 

 part of the State. 



Rocks of the Crystalline Area. 



With but few exceptions, the rocks of the Crystalline area 

 include profoundly altered, original clastic and igneous masses 

 — crystalline-metamorphic rocks. Some of the granites and 

 most, if not all, of the more recent basic dike-rocks retain in 

 the field their original characteristic massive structures. 



Many different mineralogical types of rocks are represent- 

 ed. Metamorphism has been so complete in many that it is 

 often impossible to say, with certainty, whether they were de- 

 rived from original sedimentary or igneous masses. Granites, 

 gneisses, schists, slates, limestones and quartzites or sandstones 

 compose the principal rocks. These are cut by numerous in- 

 trusions of more recent basic igneous rocks in the form of dikes. 

 Diabase, diorite and gabbro comprise the commonest types of 

 dike-rocks. Hornblende- and mica-schists are the most wide- 

 spread of the crystalline schists. The granites and a part of the 

 gneisses are mica-rocks. 



The entire Crystalline area is one of great complexity. The 

 rocks are everywhere altered, intricately folded and tilted, and 

 secondary structures induced in them. A further result of the 

 intense metamorphism is the formation of numerous secondary 

 minerals. The structural and age relations of the rocks of the 

 area have not yet been worked out. With but few exceptions 

 the rocks are geologically old, and belong to different periods 

 of formation ; some are pre-Cambrian, while others are of 

 later age. 



The Rock-Forming Minerals of the Area. 



The rocks of the area comprise most of the more common 

 rock-forming minerals and many of the rarer ones. The source 



