$26 PHYSICAL GEOGRAPHY. 



series,* a group of rocks intermediate between the Laurentian and Cambrian, and 

 exclusive of the Huronian. This proposal will relieve a multitude of difficulties, and 

 enables us to place the Atlantic system midway between the most ancient Laurentian 

 and the Paleozoic, so that both parties in the metamorphic controversy may be pleased 

 because their opponents are not altogether in the right. Furthermore, the express 

 exclusion of the New England rocks from the Laurentian, and the omission of all 

 reference to the true Laurentian areas by Featherstonhaugh in his definition of Atlantic, 

 enables us, both by a true regard for historical accuracy and correct geological discrim- 

 ination, properly to apply the latter term to the eastern belt of crystalline strata, from 

 New Brunswick, through New England, south-eastern New York, etc., east of the 

 great valley of Virginia to Alabama. 



There is a modification of the term Montalban now requiring specification. The 

 White Mountains contain of this system only the tender friable gneisses described 

 by Dr. Hunt, with the layers carrying andalusite and the Concord granite. Our 

 researches show that the rocks of this geographical area properly constitute a separate 

 division of the whole gneissic series of New Hampshire, and therefore Montalban may 

 naturally be restricted to express these and no others. The present chapter indicates 

 other modifications also. First, the porphyritic gneiss is older than any other of the 

 gneisses, and may for the present be removed from the Atlantic system, partly because 

 it may be represented in the true Laurentian, and partly because, in generalizations 

 respecting the growth of the North American continent, the Atlantic border rocks seem 

 to have had a later origin. Explorers in the Laurentian fields will do a great service 

 by ascertaining whether the porphyritic rocks cannot be separated stratigraphically 

 from the firm massive pyroxenic gneisses, which are so distinct from the tender schists 

 of the east. The porphyritic gneiss has been included in the Atlantic and Montalban 

 series by all writers in publications previous to the present. It is said to be a very 

 conspicuous formation in North Carolina, where it is regarded by Prof. Kerr as the 

 oldest of the crystalline series. Second, in New Hampshire the Bethlehem, Lake, 

 and Franconia series seem to be independent formations, separable readily for strati- 

 graphical reasons from each other, and from the Montalban. The second distinction 

 was first drawn by us in 1873, at the Portland meeting of the Association for the 

 Advancement of Science. See vol. xxii, p. 123. I am disposed now to group all 

 these older strata as follows : 



A. PORPHYRITIC GNEISS AND GRANITE, perhaps LAURENTIAN. 



B. ATLANTIC SYSTEM. 



1. Bethlehem Grotip. 



2. Lake Group. 



3. Montalban Group. 



4. Franconia Group. 



* In what I have said concerning the priority of suggestion in respect to the White Mountain series, I desire it 

 to be distinctly understood that to Dr. Hunt is due the credit of first assigning these rocks to a new system of 

 different age from any described formation, though he has unfortunately included some later rocks with them. I 



