GEOLOGY. 303 



or gneissic rocks so closely, and they are indeed so identical with them in 

 mineral aspect and structure, that the observer is baffled in his attempts to 

 distinguish the two groups lithologically; nevertheless, it clearly appears, as 

 the sections illustrating this country prove, that they are distinct systems, 

 occupying separate zones, susceptible of independent definition on the 

 geological map. 



At the time of the first construction of the general geological map of the 

 State, the true limits separating the hypozoic or gneissic from the azoic or 

 semi-metamorphic rocks were but vaguely understood, and the State geolo- 

 gist did not venture to define them on the map, but shaded the one system 

 into the other, indicating, however, what he has since proved, that the true 

 gneissic rocks, in their southwestward course, pass out of the State at the 

 Susquehanna, only a short distance north of its southern boundary, while 

 the azoic, or talco-micaceous group, as a genuine, downward extension of 

 the primal, paleozic series, widens progressively going westward, until, from 

 a very narrow outcrop at the River Schuylkill, it occupies at the Susquehanna 

 the whole broad zone south of the limestone valleys of the Conestoga and 

 Codorus streams in Lancaster and York counties. Since the revival of the 

 field work of the survey, the dividing limit of these two sets of metamorphic 

 strata has been traced and mapped with precision. To the southwest of the 

 Snsquehanna it has never, it is believed, been pursued through Maryland 

 and the other southern States, though one may readily discern it in going 

 northward or westward from Baltimore, or ascending the Atlantic slope in 

 Virginia. In Maryland it crosses the Baltimore and Susquehanna Railroad 

 about twelve miles north of Baltimore, and it is intersected by the Baltimore 

 and Ohio Railroad a little east of Sykesville; it crosses the Potomac above 

 Georgetown, and the James River in Virginia, west of Richmond. The line 

 of boundary is, however, not a simple one, but is intricately looped, in con- 

 sequence of numerous nearly parallel anticlinal foldings of the strata, send- 

 ing promontories or fingers of the older rocks, within the area of the newer 

 or semi-metamorphic, to the west of their average boundary, and causing, 

 of course, corresponding troughs, or synclinal folds of the newer, to enter 

 eastward of the average boundaiy, the general area of the older. The 

 Atlantic slope has received hitherto so little exact geological study, that we 

 are, as yet, without the data for determining with any precision, either the 

 succession of its much broken and closely-plicated strata, or the geographi- 

 cal limits which separate even the larger sub-groups. It is sufficient, how- 

 ever, for our present purpose, to show the existence and the approximate 

 range of two great metamorphic systems, separated by a physical break; 

 and the conformable relations of the later or upper of these to well-known 

 lower paleozoic formations of the Appalachian chain. 



ON THE GEOLOGICAL STRUCTURE OF THE APPALACHIAN CHAIN 



IN WESTERN MASSACHUSETTS. 



At a recent meeting of the Boston Society of Natural History, Mr. C. H. 

 Hitchcock exhibited a diagram of a geological section from Greenfield to 

 Charlemont, Mass., and gave the following explanation of it : 



This section was measured in October, 1857. The design of it is to show 

 the amount of erosion since the strata were brought into their present 

 position. 



I will enumerate the rocks in order, going from east to west, beginning at 



