116 



sixty per cent, of calcic carbonate. The bed is only a few feet 

 wide, and shows no distinct traces of stratification, but is much 

 broken and contorted ; the trend, however, is north-easterly. 

 The enclosing rock is likewise entirely devoid of apparent strati- 

 fication, consisting mainly of diorite and hornblendic granite, 

 with limited masses of petrosilex. Yet I am convinced that this 

 dolomite belongs to the stratified group, and is contemporaneous 

 with the associated rocks ; in other words, its mode of occurrence 

 admits and necessitates the same explanation that was offered in 

 the case of the similarly isolated masses of quartzite. 



I have not examined the limestones in Concord and Natick ; 

 according to Prof. Hitchcock, however, they are magnesian, 

 non-serpentinic, probably stratified, and intimately connected 

 petrologically with the stratified group. 



GENERAL RELATIONS OF THE HURONIAN ROCKS. 



Having proved that the stratified group, including the lime- 

 stone, and the eruptive diorites, taken together, constitute one 

 great basic or dioritic series, having the terranes of the former 

 group mainly at the top and of the latter at the bottom, the 

 inquiry is now in order as to what are the relations of this 

 basic series to the petrosilex and granite. In the first place I 

 would observe what might, perhaps, have been better noted 

 before, viz., that the petrosilex and granite form another great 

 series, structurally parallel with the dioritic series, but in strik- 

 ing contrast with it chemically ; the one series being, as we 

 have seen, decidedly basic, while the other is, as a whole, highly 

 acidic. The typical and euritic granites, consisting mainly of 

 orthoclase, but containing much quartz and poor in hornblende, 

 are, of course, types among acidic rocks. It is true that the 

 rocks described under the head of petrosilex include much fel- 

 site, which, according to our definition, is basic ; yet petrosilex 

 proper is by far the prevailing species and our petrosilicious 

 rocks, taken as a whole, are unquestionably acidic, although 

 the silica ratio is probably less than among the granites. In 

 characterizing this acidic series as structurally parallel with 



