GEOLOGY. 15 



descent is much more precipitous. The western boundary of 

 the primary is not very accurately established, but it is believed 

 to lie not far from a line running nearly N. and S. through the 

 centre of Gilmer county, and continued in the same direction 

 to near Canton in Cass county, thence to the western base of 

 the Allatoona mountain on the Etowah river, where it turns to 

 the S. W., and passing near Van Wert in Paulding county, and 

 along the northern base of the Dugdown mountain to the Ala- 

 bama line. 



The N. W. part of the State, bounded to the east and 

 south by the western limit of the primary, consists of a transi- 

 tion or older fossiliferous formation, except the extreme N, W. 

 corner where the carboniferous occurs. 



Having thus briefly sketched the great leading features of 

 the geology of the State, the various formations will now be 

 described more in detail. 



Tertiary Formation. — The sea-coast of Georgia is rich 

 in the more recent formations. In the salt-marshes and 

 swamps, which are spread out between the sea islands and 

 the main land, and along the borders of the rivers and creeks, 

 are extensive bodies of recent alluvium. 



Although these deposits come under this general head, they 

 are obviously so different in age as to admit of being subdi- 

 vided with advantage. The most recent is that constituting 

 the salt-marsh and the tide-swamp. This is a very modern 

 alluvium, still in the course of formation from the deposits of 

 sedimentary matter brought down by the rivers, or the reflux of 

 the tides. It consists of a tenacious blue clay, mixed with 

 fine silicious sand and vegetable matter : and at the depth of 

 from ten to twelve feet rests on a sandy post-pliocene forma- 

 tion. This subdivision of the recent alluvium contains no fos- 

 sils, except of such animals as now exist in it. The older 

 subdivision forms the inland swamps above the reach of tides, 

 and occurs not unfrequently in tide swamps, in the form of 

 small knolls, and in the salt marsh sometimes rising above its 

 surface, but generally underlying it at the depth of from three 

 to four feet. It consists of a very compact clay, destitute of 

 vegetable matter, varying in colour, but most usually blue or 



