NEW HAMPSHIRE. 395 



neglect to invoke all the inversions and foults that are reqnireil for truthful 

 elucidation." (Geology of New Hampshire, II., i)p. G58, 059. See also Am. 

 Jour. Sci., 1877, (3) XIV., pp. 310-321 ; 1878, XVI., pp. 399-401.) 



Since it was evidently the case on the Xcw Hampshire Survey that lith- 

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

 tion of the age 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. Wadsworth, of one of the collections of the New 

 Hampshire Survey, obtained from the State Geologist, may therefoi'e be 

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



As an example of the value of the lithological 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 " Preliminaiy Cata- 

 logue." This rock is said by Prof. Hitchcock to be lithologically the 

 same as the country-rock of the Men-imac mine at Xewburyport,* and 

 the hornblendic granite of Gloucester and Quincy, Mass. All these are 

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

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

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

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

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

 and the probable occuiTence of similar metalliferous deposits. (See Ge- 

 ology of New Hampshire, I., p. 27 ; II., pp. 22, 630 ; 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 case 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 lithological resemblance to each other. 



The Quincy and Gloucester (Cape Ann) hornblendic granites (syenites) 

 •were 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 careful examina- 

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

 granite, the only difference between the two being that lepidomelane 

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

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



* Tlie real location of the Merrimac mine is, however, in Newbuiy, not Xew- 

 buryport. 



