538 PHYSICAL GEOGRAPHY. 



Vershire, Vt, and those described in Hanover, Lebanon, etc., prove 

 that a more concentrated sulphate ocean existed in this than in the 

 Huronian period. 



In the White Mountain region there are several small areas of andalu- 

 site slate, supposed to be the equivalent of the Coos group. They occur 

 upon the east side of Mt. Washington, Mts. Willard and Tom, Mt. Pe- 

 quawket, and farther south in Farmington and Rochester. These facts 

 seem to indicate that the White Mountains were considerably depressed 

 during the Coos period, probably more so than the Green Mountains, 

 since no slaty beds of this age have been found resting upon the Mont- 

 alban or Huronian areas of Vermont. 



The Coos period was terminated by eruptions of sienitic granite. The 

 conical Mt. Ascutney is the best example of this igneous material. 

 Others are Black mountain, Dummerston, the Washington and Essex 

 county mountains, Vt., Iron-ore hill, near Haverhill, and the singular 

 concretionary granite of Craftsbury, Vt. The Coos quartzite now con- 

 stitutes a distinct range of mountains. 



At some unknown epoch, posterior to the consolidation of the andalu- 

 site slates, there was a considerable eruption of igneous material, pro- 

 ducing Mts. Mote and Pequawket. The paste, cementing together the 

 slaty fragments, bears some resemblance to the Albany granite, except 

 in the abundance of dark spots commonly present. I think the rock on 

 the top of Mt. Willard will probably turn out to belong to the Mt. Mote 

 series. 



The Helderberg Period. 



For a very long period of time New Hampshire furnishes no indica- 

 tion of geological changes. Our next formation was deposited at the 

 very close of the Silurian era. Fossils have been found which identify 

 the strata with those of the Helderberg mountains in New York. 



The ocean must have retired from the Connecticut valley after the 

 deposition of the Coos rocks, — otherwise there would have been formed 

 ledges to indicate the fact of continued submergence. The Helderberg 

 ocean probably covered the same area with the one just described west 

 of the Atlantic ridge, bordering the quartzite. The Helderberg rocks 

 occupy isolated areas in Bernardston, Mass., Hanover, Lyman, Lisbon, 



