AZOIC ROCKS. 455 



between granitic gneiss and granite is but slight. For if gneiss by metamorphie action may become 

 granite (or the reverse, according to Scrope), may fuse into it, as it 'were, the stage of the process 

 immediately preceding the granite state, cannot be far removed from the granite itself. Examples of 

 granitic gneiss may be found at the village of Jamaica, and in the greater part of the gneiss along Connecti- 

 cut River. It is a question whether the so-called granite between Waterford and Maidstone, is other than 

 granitic gneiss. 



III. HORNBLENDIC OR AMPHIBOLIC GNEISS. 



Hornblende is exceedingly common in gneiss. When the amount of hornblende present is not great 

 enough to constitute hornblende schist, but enough to give character to the rock, we call it hornblende 

 gneiss. The hornblende is usually disseminated in black foliated masses through the gneiss. It is most 

 abundant in the vicinity of hornblende schist ; and that is most abundant in the south part of the State. 



IV. PORPHYMTIC GNEISS. 



The peculiarity of this variety consists in the method in which the feldspar is distributed through the 

 rock. It occurs in rounded or ovoid masses, like small pebbles. These masses are generally a part of a 

 single crystal, and are permeated by the normal cleavage of the orthoclase feldspar. Often the rounded masses 

 would be mistaken at a distance for pebbles. This variety is poorly developed in Vermont. The localities 

 where it is most conspicuous are in Weathersfield and Grafton. 



V. EPIDOTIC GNEISS. 



This variety is not abundant in Vermont. It usually contains some hornblende as well as epidote. It is 

 the green variety of epodite that is thus associated, and the gneiss is accordingly tinged slightly with this 

 color. It is found chiefly in the gneiss along Connecticut River. 



VI. CALCAREOUS GNEISS. 



Wo cannot say how much of the gneiss contains carbonate of lime. It is generally the case that the 

 layers of gneiss adjoining beds of saccharoid limestone are largely composed of this mineral, but it is not 

 generally supposed that there is much lime in the gneiss elsewhere. Almost by accident we tested a suite 

 of specimens of the gneiss in Mount Holly and Walh'ngford, upon Section V, and found that nine out of 

 twenty-tour specimens were calcareous. In general these specimens were not from the vicinity of beds of 

 limestone. But it is remarkable that so many of them should be calcareous. Whether specimens from 

 other localities would show a large per cent, of the calcareous varieties, remains to be proved. We think it 

 doubtful whether the lime is abundant elsewhere. If lime is abundantly ditVused through the gneiss, it is 

 remarkable that it should not have been developed in the form of lime feldspar, as it has been in the Lau- 

 rent ian n>cks. The inference has been drawn, that the gneiss of Vermont is not Laurentian because the 

 lime t'eldsjvir is wanting. But if the lime should be present in the rock, though in another form, would it 

 not be sutlicient to identity the gneiss in different localities, so far as can be done by the aid of their chem- 

 ical composition? We are in great danger, however, of using chemical aud lithological characters in 

 distinguishing strata, as if they were as consequential as organic remains. 



VI I. TALCOSE GNEISS. 



This is composed of feldspar, quartz, and the so-called talc of the talcose schist of Vermont. Or it may 

 be gneiss with taleky seams. Kxamplcs of it may Iv found in the State Cabinet, from North Londonderry. 



vm. FKi.nsr.vu vxn gi .vurz. 



A largo part of the gneiss is deficient in mica, and is, therefore, composed of feldspar and quartz. It 

 occurs in thick-l>edded masses, semi-granitic; or it may occur iu thin layers, ami Iv quite fine grained. 



