GEOLOGY. 453 



enic eruptive rocks. Mr. Dodge there distinguishes an older (the Brain- 

 tree) and a younger (the Quincy) syenite, the latter holding numerous 

 fragments of a fine-grained black diabase, as well as fragments of a 

 tine-grained syenite, but it is not clear that any of these are derived 

 from the older syenite. The younger of these overlies the slates, and 

 the older would also appear to be more recent than these, but the 

 point is not clearly brought out. The slates in the vicinity of the intru- 

 sive masses are altered, and present large oval cavities partially filled 

 withepidote, sometimes irregularly distributed and sometimes scattered 

 along discolored bands parallel to the stratification. The writer has 

 observed similar conditions elsewhere in the slates of the Boston basin 

 in proximity to intrusive rocks. 



SILURIAN, CAMBRIAN, AND TACONIAN. 



I. C. White, of the second geological survey of Pennsylvania, has 

 described the unconformable superposition of the Oneida sandstone, 

 the base of the (true) Silurian, to the Hudson-Biver group, as well seen 

 on the Erie railroad near Otisville, N. Y., the dip of the former being 

 there 28° and that of the latter 43° to the north, while the lower series 

 has an eroded surface, and has, moreover, furnished fragments to the 

 overlying sandstone. Similar evidences are seen at the Lehigh Water- 

 gap. This stratigraphical unconformity, though sometimes questioned, 

 was long since pointed out by H. D. Rogers, and was confirmed by the 

 present writer in 1878. In this connection, there arises an important 

 question as to the geological position of the so-called Hudson-Biver 

 slates. The evidence from the valleys of central Pennsylvania, as well 

 as on the north shore of Lake Ontario, is that there is no unconformity 

 nor stratigraphical break between the Oneida sandstone and the Lo- 

 raine shales, which are often regarded as identical with the Hudson- 

 Biver slates. As has, however, been pointed out by the present writer, 

 the Hudson-Biver group, as at first proposed by Vanuxein, was by him 

 made to include not less than three distinct groups of argillaceous 

 strata, two of which he declared to be — in eastern Pennsylvania, at 

 least — geographically distinct, namely, the fossiliferous Loraine shales 

 of the central valleys, there lying conformably beneath the Oneida 

 sandstones, and the non -fossiliferous argillites of the great Appalachian 

 valley, which, as described above, are unconformably overlaid by this 

 same Oneida sandstone. The Loraine shales, according to Hunt, are, 

 so far as yet known, unrepresented in the great valley, where, however, 

 besides the roofing-slates belonging to the Lower Tacouic — the Trans- 

 ition Argillite of Eaton — there is in many places a great development 

 of red sandstone, conglomerates, and argillites, the continuation in 

 Pennsylvania of the First Graywacke of Eaton, which from southern 

 New York is traced east of the Hudson, and thence to Quebec and 

 beyond. This is the Cambrian of the Appalachian area, the Upper Ta- 

 conic of Emmons, and the Potsdam and Quebec groups of Logan, which, 



