4 st; 



Report of the State Geologist. 



the different varieties of rock included under it, so constantly shade into 

 each other, or are so intimately associated as to render their precise differen- 

 tiation difficult or impossible. 



In composition, the rocks range from acidic, like granite, to basic, like 

 gabbro. In structure they range from fine to very coarse, and from distinctly 

 gneissoid, or even schistose, to entirely massive. The prevailing type, in the 

 region here covered, is a fairly acid, light-colored rock, of medium or fine 

 grain, and rather obscure foliation. It often looks as though its fine grain 

 had resulted from a crushing of larger constituents. Other important 

 varieties are : a coarse augen-gneiss and a porphyritic gneiss often extremely 

 coarse, and nearly or quite massive. The dark-colored basic varieties seem to 

 be much more limited in extent, the only large area noted being in South 

 Russell. Brief accounts of some of these varieties have been given elsew 'here, 

 and need not be repeated, as such details as are necessary will be brought out 

 in discussing the origin of the rocks. 



When this latter question is considered, three possible explanations 

 present themselves as being worthy of careful consideration. According to 

 one of these, the gneisses are metamorphosed sediments; according to the 

 second they are of ingneous origin ; while the third comprises both of the 

 others, regarding the gneisses as in part sedimentary and in part igneous. 



Dealing with the internal evidence alone, the parallel structures of the 

 gneiss afford the only support for the hypothesis of metamorphosed sediments. 

 And when the evidence is examined it appears very weak, for, as a rule, there 

 is only a foliation which could not be regarded as in any way connected with 

 original bedding, but is clearly a secondary structure, and often very incon- 

 spicuous. Distinct banding that might represent original stratification is rare, 

 and such bands as do occur are generally widely scattered and of limited 

 extent, demanding, as show n below, a different explanation. Indeed, rather 

 than showing pronounced banding, the rocks are very uniform in texture and 

 composition over wide areas, and where they change, do so gradually. It 

 may be safely stated that there is nothing in the structure of the gneiss as a 

 w hole, w hich demands, <>r even suggests, that it should be regarded as derived 

 from a sedimentary formation. On the contrary, the complete absence of 

 clastic structure, the lack of any trace of stratification, and the uniformity over 

 wide areas are all opposed to such a view. 



The chief support for this hypothesis is found in the limestone associated 

 with the gneiss. This is doubtless a metamorphosed sedimentary rock audits 

 presence in long belts, whose trend is parallel to the foliation of the gneiss, 



