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PHYSICAL GEOGRAPHY. 



The Mica Schist Periods. 



The fifth of our map illustrations represents by one color all the for- 

 mations that have been deposited in New Hampshire to the close of the 

 Huronian; the other shows the positions of the areas occupied by- 

 several groups which were evidently deposited subsequently, and may 

 belong to one great system. They are thus placed, however, provision- 

 ally. They are the Rockingham mica schist, the Merrimack group, 

 Cambrian, auriferous clay slates, and the Coos group. 



The Rockingham schist occupies the principal portions of Rocking- 

 ham, Strafford, and Belknap, with a part of Merrimack and Hillsborough 

 counties. Over this area it is generally an uncouth mica schist. West 

 of the Merrimack river there are several long, narrow areas of mica 

 schist supposed to belong to the same group. A broader range, extend- 

 ing north-easterly from Deering through Henniker and Hopkinton, is 

 highly ferruginous. A similar band occupies most of Weare and Fran- 

 cestown. A cleaner mica schist makes up the substance of the moun- 

 tain range extending from New Ipswich through Sharon, Temple, and 

 Lyndeborough. All these areas cover Atlantic gneisses unconformably. 

 They lie upon them like a blanket, and hinder us much in our attempts 

 to study the older formations. The Merrimack group is a micaceous 

 quartzite lying adjacent to the Exeter sienite range on the north-west, 

 and has not yet been separated from the previous group. The occur- 

 rence in it of large beds of soapstone, as at Groton, Mass., is suggestive 

 of the Huronian age. It abounds in beds of coarse indigenous granite, 

 which seem to have been altered in situ from feldspathic conglomerates. 

 In certain parts of Strafford county these granite beds predominate, 

 forming numerous hills, while the slate occupies the valleys between. 



The schists south-east of the Exeter sienite range may belong to the 

 same Merrimack era ; but they are traversed by several narrow bands of 

 clay slate. Possessing a north-west strike, these slates are thought to 

 be the equivalents of the Paradoxides beds of Massachusetts. It is obvi- 

 ous that the physical aspect of the country must have been very different 

 from what it was in the Merrimack period. The Merrimack schists must 

 have been elevated, then they were cut across by streams flowing south- 



