260 STRATIGRAPHICAL GEOLOGY. 



logical resemblance to the lower Skiddaw slate of the Cumbrian moun- 

 tains in England. These are Cambro-Silurian. 



Prof. Sedgwick divides the Cumbrian rocks of northern England into 

 three parts: (i) Skiddaw slate or Lower Cumbrian; (2) chloritic slate 

 and porphyry, or Middle Cumbrian ; (3) Coniston group or Upper Cum- 

 brian. These arc regarded as the equivalents of the Cambrian, from the 

 Longmynd to the Bala group, or, in this country, from the Potsdam slates 

 to the Trenton. 



Concerning the Skiddaw slate, wc have the following language from 

 the introduction to a systematic description of the British Paleozoic fos- 

 sils in the geological museum of the University of Cambridge (p. Ixxxiii, 

 1855): 



Immediately over the central granite of the Skiddaw forest we have a very co.npli- 

 cated metamorphic group, superior, perhaps, in importance to any other metamorphic 

 group in South Britain. It has mineral veins which, though apparently of a much 

 older epoch, present very interesting analogies to the mineral veins of Cornwall. This 

 subgroup passes almost insensibly into the ordinary lower Skiddaw slate, through the 

 intervention of a chiastolite rock and a porphyritic chiastolite slate. 



7. PorpJiyry. This rock often passes insensibly into the feldspathic 

 part of the Chocorua group. Dykes of it cut the porphyritic gneiss, all 

 the divisions of the Atlantic group, and the Huronian. In one of our 

 annual reports it was ranked with the upper Huronian, because it resem- 

 bled closely the stratified porphyries referred to this series in Massachu- 

 setts, New Brunswick, Ontario, etc. It seems best now to call it eruptive, 

 since so many dykes of it are found in the White Mountain district. Its 

 period of eruption is thought to have preceded that of the Pemigewasset 

 granites, because fragments of the porphyry are contained in the Albany 

 granite, on Mts. Pequawket and P'lume, and in the Conway on Cascade 

 brook. 



8. Pemigcivasset Granites. These have cut the andalusite slates, and 

 are therefore of later origin, whether that be Eozoic or Paleozoic. The 

 theory of an overflow may not be tenable, — certainly it is not applicable 

 to certain portions of the field occupied by the granites. 



Two additional sections crossing these granites have been added upon 

 Plate XI, in order to illustrate their surface outcrops and supposed inte- 

 rior connections. Fig. 25 shows the rocks between Mt. Lafayette and 



