1884.1 NATURAL SCIENCES OF PHILADELPHIA. 24t 



their relative age, is a point upon which there has been mnch 

 difference of opinion, owing to the confusing arrangement of the 

 strata throughout some of the counties of southeastern Pennsyl- 

 vania. Notwithstanding such difficulties, the geologists who have 

 discaided broad generalizations, and devoted themselves to the 

 study of local details, find the way gradually opening to a better 

 understanding of the truth ; and the early presumption that the 

 mica schists and gneisses are of Paleozoic age, is rapidly becoming 

 a matter of general acceptation. The latest results of geological 

 study in Pennsylvania, together with the observations of the 

 writer throughout Northern Delaware, tend to show quite con- 

 clusively that the crystalline rocks represent the following ages : 

 (1) The Laurentian (?) including the rocks of the hornblendic 

 belt ; (2) the Potsdam, to which the quartzitic and sand rocks 

 belong; (3) the Calciferous, including the magnesian marbles, 

 and (4) the age of the mica schists, which must be placed some- 

 where above the Trenton, and, according to Mr. Charles E. Hall, 

 above the Hudson River, slates. With these points in view, we 

 shall proceed with the demonstration. 



The Laui'entian (?) — The so-called Laurentian area of Delaware 

 is but a continuation of identical areas in southeastern Pennsyl- 

 vania, within the Philadelphia belt, which, according to Mr. Hall, 

 are three in number, each connected with the other by a narrow 

 neck, and each situated successively to the northeast. The 

 southernmost of these areas is of lenticular shape, reaching from 

 Chester Creek, and spreading out over the southern part of Dela- 

 ware County, whence it extends into the State of Delaware, as 

 shown upon the map. Northeast of this patch, and to the north- 

 west of Media, is another area of irregular form, while a third — a 

 long east and west belt — runs from West Chester, south of Con- 

 shohocken, eastward to the Delaware River, near Trenton. Thus 

 the Delaware Laurentian forms the southernmost tongue of the 

 one Laurentian area of the Philadelphia belt, the upper portion 

 of which has been for a long time known as the Third Belt of 

 Rogers. As regards the age of the Third Belt, Mr. Hall says, 

 " The rocks of the Third Belt are identical with the granitoid and 

 syenitic rocks of the Welsh Mountain, north of the Chester 

 County limestone valley. These rocks of the Welsh Mountain 

 are similar in all respects to the crystalline rocks extending into 



