230 STRATIGRAPHICAL GEOLOGY. 



CiiocoRUA Group. 



The study of the sienites and other rocks from Albany shows that our 

 Chocorua group may be divided lithologically into three parts: — First, 

 the crystalline aggregation of coarse orthoclase crystals, both alone and 

 mixed with quartz, containing, also, mica in a few localities, so as to be 

 properly a granite. The feldspar is often greenish in color, and was 

 known by us, when exploring, as the "green granite," such as occurs 

 about a mile above the mouth of Hancock Branch (No. s%). The second 

 variety is a fine-grained, grayish-green, compact potash-feldspar. This is 

 common about Mt. Chocorua, particularly in the ravine on the east side 

 of the pinnacle. This variety merges into a porphyry, and both of them 

 pass into the Albany granite, such as constitutes Sawyer's Rock. None 

 of the kinds mentioned is so easily confounded with labradorite as this. 

 The third division is the sienite, whose particular distribution has just 

 been described. Whether these three kinds really belong to different 

 eruptive periods, or should be properly grouped as one, is a question for 

 the future never suggested before this moment of writing. No attempt 

 will be made to distinguish them upon the map. 



CJiocoriia group in Vermont. Upon page 529, Volume I, it is stated 

 that there is a small area of Chocorua granite at Cuttingsville, upon the 

 Rutland & Burlington Railroad. The second and third of the above 

 named varieties are present there. Although the mineral is not labra- 

 dorite, as there stated, the argument for the age of the Green Mountain 

 rocks is valid, since there is a close agreement between the two gran- 

 ites and their associations. In the briefest terms possible, the argument 

 may be stated thus : Along the axis of the Green Mountains there occurs 

 a large overflow of granite, apparently identical with the Chocorua granite 

 of the White Mountains. In a limited region igneous rocks, having the 

 same mineral composition, must have been ejected from one vast caldron 

 of melted material, or at least at the same epoch. Therefore the granites 

 of Cuttingsville are of the same age with those of Mt. Chocorua. Fur- 

 thermore, the Chocorua rocks cut formations of Eozoic age; — therefore 

 the strata disturbed by the Cuttingsville outburst are of Eozoic date, or 

 older — they cannot be newer. And, as the rocks about Cuttingsville be- 

 long to the Green Mountain gneiss, that whole system must be as old 



