xcvi GENERAL SUMMARY OF SCIENTIFIC AND 



ain between the latter river and the Susquehanna, as clearly of Lau- 

 rentian age, and has referred to the Montalban, the Philadelphia 

 gneiss and mica-slate series. He points out the existence of a hither- 

 to unnoticed Laurentian belt farther to the southeast, which sepa- 

 rates the Auroral limestones on that side of the Mesozoic from the 

 Montalban series ; and he notices between the latter and the Lauren- 

 tian a belt of Huronian strata of varying breadth, which is probably 

 unconformable with the Laurentian. He moreover describes the 

 South Mountain range, where it reappears again to the south of the 

 Susquehanna, as consisting, in Pennsylvania, chiefly of a large de- 

 velopment of Huronian strata, including great interstratified masses 

 of orthofelsite or petrosilex rocks (the halleflinta of the Swedish 

 geologists), which frequently pass into a quartziferous feldspar- 

 j)orphyry, generally of some reddish color. These acidic rocks rep- 

 resent in the Huronian the gneisses of the Laurentian and the Mont- 

 alban, and here, as elsewhere, are found to be distinctly stratified, 

 and intercalated with greenstones and chloritic and epiclotic rocks. 

 He regards them as the equivalents of similar rocks which form a 

 part of the Huronian series along the eastern coast of New England 

 and New Brunswick, and of the iron-bearing orthofelsite porphyries 

 of Missouri. They are also developed along the northern shore of 

 Lake Superior, and their ruins enter largely into the conglomerates 

 of the Keweenian series in Northern Michigan. 



LOWER TACONIC ROCKS. 



Hunt has further described the characters of the Primal and Au- 

 roral rocks of Rogers, as seen in the region to the southeast of the 

 North Mountain in Pennsylvania. These rocks were assumed by 

 Rogers to be the stratigraphical equivalents of the Potsdam, Calcif- 

 erous and Chazy of the New York series, and of the undoubted rep- 

 resentatives of these which occur west of the North Mountain. 

 They are the Lower Taconic quartzites and limestones of Emmons, 

 maintained by him to occupy a lower horizon than the base of the 

 New York fossiliferous series. They constitute, according to Hunt, 

 a partially crystalline series, distinct from and newer than the Hu- 

 ronian and Montalban. Interposed in the detrital rocks of the 

 Primal are schistose beds charged with micaceous and chloritic mat- 

 ters, crystalline limestones often inclosing serpentine, and important 

 beds of magnetite, as at Boyertown, Cornwall, and Dillsburg, all 

 of which present mineralogical characters unlike those of the un- 

 derlying crystallines. Similar conditions extend into the Auroral 

 magnesian limestone, giving to these two formations a distinctive 

 character. They were evidently deposited over a subsiding conti- 

 nent with bold shores, so that while the Primal has in some places a 

 very great thickness, it is elsewhere very thin or entirely wanting be- 

 neath the Auroral, which rests directly upon the older crystalline rocks. 



