NEW HAMPSHIRE. 



Q( 



95 



iie<4ect to invoke all tlie inversions and faults that are retpired for truthful 

 elucidation." (Geology of New Hampshire, II., pp. G58, (559. Sec also Am. 

 Jour. Sci., 1877, (3) XIV., pp. 316-321 ; 1878, XVL, pp. 309-401.) 



Since it was evidently the case on the New ITampshh'e Survey that lith- 

 ologlcal characters were considered to be all-important in the determina- 

 tion of the ago of the crystalline rocks, it seemed desirable to find out 

 what amount of skill had been shown in ascertaining what the character 

 and true names of these rocks really were. A few results of a partial 

 examination, by Dr. AYadsworth, of one of the collections of the New 

 Hampshire Survey, obtained from the State Geologist, may therefore be 

 here introduced for the purpose of throwing light on this question. 



As an example of the value of the hthological determinations of this 

 Survey, the typical Exeter syenite may be first taken. The specimen is 

 No. 71 of Hawes's Catalogue, and No. 200 of the "Preliminary Cata- 

 logue.'* This rock is said by Prof. Hitchcock to bo lithologically the 

 same as the country-rock of the Morrimac mine at Ncwburyport,* and 

 the horublendic granite of Gloucester and Quinoy, Mass. All these are 

 regarded as being probably of Laurentian age, and it is recomcnded that 

 search be made in the Exeter rock for mineral veins, on account of its 

 resemblance to that in which the Merrimac mine is found. The theo- 

 retical idea at the base of this piece of advice seems to be, that, even in 

 eruptive rocks, identity of iithological character indicates identity of age, 

 and the probable occurrence of similar metalHferous deposits. (See Ge- 

 ology of New Hampshire, I., p. 27 ; IL, pp. 22, G30 ; III., Part V. p. 34.) 

 It has by no means 'been established, as the result of observation, that 

 rocks of the same geological age and mineralogical composition contain 

 the same useful ores ; but in the present ease it is not necessary to go 

 so far as this, since it can easily be shown that the rocks to which ref- 

 erence is here made have no Iithological resemblance to each other. 



The Quiucy and Gloucester (Cape Ann) horublendic granites (syenites) 

 ^vcrc described by Dr. Wadsworth in 1878 (Proc. Bost. Soc. Nat. Hist., 

 XIX., pp. 309-316). They consist of grayish, granitoid, coarsely crystal- 

 line aggregates of feldspar, quartz, and hornblende. In some places this 

 granite has a reddish color. This rock was found, on carefiil examina- 

 tion, at Ilockport, to pass, in the same continuous mass, into a micaceous 

 pmite, the only dilFereuce between the two being that lepidomelane 



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h, 



replaced tlie hornblende. The feldspar proved to be mainly orthoclase. 

 The country-rock of the Merrimac mine, on the other band, is a dark- 



* The real location of the Mummac mine is, however, in Newbury, not New- 

 bury port. 



