892 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



*' 4. Hnronian. The talcose scliist series along Connecticut Riv(^r, and in 

 the north part of Coos County. 



" 5. Older Cambrian ? Includes Coos and Merrimack groups, and probahly 

 the * Calciferous Mica Schist ' of Vermont Survey. 



"11. Paleozic. 

 " Helderherg limestones. 



*' Clay slates." 



In discussing these formations it is later stated as follows : 



** If these granites behave like a stratified formation, of course the question 

 is at once raised whether they should not be regarded as true strata. The 

 answer cannot be given from position merely, since it is not uncommon to find 

 sheets of trap or lava holding a perfectly analogous position. We have pre- 

 ferred to think of the White Mountain country at the end of the Laurentian 

 period as an immense basin, upon which there was an overflow of common 

 granite. Being liquid, it spread itself out like water, assuming a horizontal 

 surface. After a while there was an eruption of trachytic granite, which spread 

 itself in the same way. Subsequently the felsites were formed above them 

 conforTnably. It would be natural to regard these granites, and felsites as 

 belonging to one period, the Norian. The limits of this system have not been 

 fixed ; and it seems as if in New Hampshire it should commence in the com- 

 mon granite, and end with the red orthoclase felsite 



" If the felsite series is of the age of the Upper Laurentian or Labrador of 

 Logan, then by the law of superposition the strata underneath the common 

 granite are Lower Laurentian. Observation showed us, at this phase in the 

 development of the White Mountain structure, two gneisses and a breccia 

 underneath the granite sheet. The most important is the * Forphyritic gneiss,' . 

 or granite sometimes. This is a gneiss having large crystals, usually one and 

 a half inches long, of orthoclase, arranged in layers in the mnss, with the longer 



axes parallel to one another. These we conceive to be the strata 



" The description of the Laurentian rocks in Canada and Europe make men- 

 tion of large quantities of porphyritic gneiss; hence we feel warrani ed in refer- 

 ring these lower schists to the Laurentian system. We have yet fois ud nothing 



older in the state 



" Our explorations have brought to light the existence of ten distinct periods, 

 whose records caii be traced upon the scarred sides of these highest mountains 

 of New England 



" If our limited opportunities have led to such unexpected results, what 

 may we not look for when the geological structure of the entire metamorphic 

 area of New England has been carefully studied ! " (Proc. Am. Assoc. Adv. 

 Sci., 1872, XXI. pp. 134-151.) 



In 1873 Prof. Hitchcock remarks that the evidence of the inferior 

 position of the White Mountain or andalusite gneiss 



" to the Labrador group is very decided In the valley of Dry or Mt, 



