Campbell — Rogers's Geology of the Virginias. 359 



illustrate our geology in a comprehensive way, and will be a 

 most valuable help to a clear comprehension of the subject be- 

 fore us. But the younger field- workers at least, some of whom 

 doubtless desire to see for themselves what we have in this 

 wide and varied field of 61,400 square miles, will be aided in 

 their explorations by brief sketches of some characteristic feat- 

 ures of our several geological formations and a statement of 

 points on leading routes of travel at which observations can be 

 conveniently made. We therefore offer a general view of each 

 of the geological formations and groups found in the Virginias, 

 taken in their chronological order, which is not done in the 

 volume ; except so far as relates to a very concise view given 

 of the several subdivisions of the Appalachian range in the 

 second and third annual reports. 



The notation by numbers from I to XYI, given by Prof. 

 Eogers, will be retained in our present discussion as a part of 

 the history of the survey, and at the same time we shall give 

 the more complete and generally applicable notation of Prof. 

 Dana's table, which has been practically adopted by Prof. 

 Eogers in the last chapter of the volume, page 717. 



It may be well to state in this connection that a brief sum- 

 mary of many of the most important facts given below will 

 appear in our notes on Virginia prepared for a forthcoming 

 new edition of Macfarlane's Geological Railway Guide. 



The reader of Prof. Rogers's Reports will find some few mis- 

 takes and inconsistencies which no doubt would have been 

 corrected had he lived to see his work pass through the press. 

 But we must remember that he had to begin on a very wide 

 and unexplored field, that he was laying an important part of 

 the foundation of our grand system of American Geology, the 

 superstructure of which has been in process of erection for half 

 a century and more, and is not yet completed. In this prelim- 

 inary work it is astonishing to find so few errors in the pub- 

 lished results of his labors, and to find what a solid basis his 

 successors have to build upon. 



Archaean Group. — This group constitutes the country rocks 

 over an extensive area in Virginia, but is found nowhere in 

 West Virginia except in the N.E. corner near Harper's Ferry. 

 To these rocks Prof. Rogers in his State Reports applies the 

 old name, "Primary;" but in his notes for Macfarlane he 

 adopts the term, "Archaean." His map shows very clearly 

 the outline of this great geological belt, extending from the 

 Potomac to the line of Tennessee. Its eastern boundary is a 

 somewhat waving line extending from the city of Alexandria 

 in a southern direction through the cities of Fredericksburg, 

 Richmond and Petersburg, and cutting the line of North Caro- 

 lina a little way north of Weldon. From that point its south 



