118 



The true sequence, then, of our Huronian terranes is as fol- 

 lows : granite, petrosilex,diorite, and stratified group ; and coin- 

 cident with this lithologic series we have a gradual passage from 

 highly acidic rocks to those that are almost entirely composed 

 of basic minerals. This chemical gradation has long been 

 recognized as essential to the constitution of every complete 

 series of sedimentary rocks, representing one prolonged period 

 of subsidence or elevation of the ocean bed ; and Dr. T. Sterry 

 Hunt has correlated this theoretically invariable succession of 

 the strata with the different degrees of mobility possessed by 

 the various products of the decay of crystalline rocks and the 

 order observed in their removal. The same authority, reason- 

 ing from well-established chemical principles, has demonstrated 

 that, from the period of the first-formed sediments on the globe 

 to the present day, the proportion of the alkalies potash and 

 soda in the rocks of the successive ages must, on the average, 

 have steadily diminished ; and each complete lithologic series, 

 each rounded cycle of sedimentation, is an ej)itome in this 

 respect of the vast cycle of which it forms but one of the stages. 

 Now this principle is well exemplified in the Huronian system 

 of Massachusetts, for in going up from the granites at the base 

 of the system, through the petrosilex, hornblendic gneiss, and 

 diorite, to the more or less micaceous beds at the summit, we 

 find a gradual diminution in the proportion of potash and soda 

 which the rocks contain. 



I have no data upon which to base trustworthy estimates of the 

 thickness, either absolute or relative, of the different members 

 of the Huronian system. I can only assert positively that the 

 aggregate volume is something enormous, and that the acidic 



damental division of the Huronian series of this region here insisted upon. He says, 

 Vol. II., p. 669, " It appears that there are two well-marked divisions of this system, 

 the upper quite chloritic, and the lower quartzose and feldspathio. The greenstones of 

 our State seem to be closely allied to the upper Huronian, and the porphyries of Lynn, 

 etc., Mass., to the lower division. It is possible that our supposed eruptive porphyries 

 of the White Mountains belong to this lower division. They certainly possess the same 

 lithological features, and in some cases have been protruded through what we consider 

 the upper division." 



