1 86 MINERALOGY AND LITHOLOGY. 



abundant, both in the ground mass and among the porphyritic crystals; 

 in others, it is nearly absent from both. In one variety, feldspar crystals, 

 with the basket-work structure characteristic of microcline, are found. 



In Albany there is a red quartz porphyry, in which some of the feld- 

 spar crystals are penetrated through and through by apatite, the needles 

 of which run into the ground mass in some places, but are not found in 

 the quartz. 



On Twin mountain a very light gray quartz porphyry occurs, in thin 

 sections of which the microscope reveals the presence of some light-col- 

 ored garnets. These garnets are entirely irregular and formless, like the 

 garnets found in granite. 



A porphyry breccia or porphyry conglomerate occurs at Waterville. 

 This is a rock which is entirely composed of pebbles or angular frag- 

 ments of red quartz porphyry, that have been cemented together into a 

 very firm, hard rock. 



From these descriptions, I think it will be plain that the quartz por- 

 phyries of New Hampshire form a group of rocks, which, though remark- 

 ably uniform in general characteristics, are remarkably diversified in 

 their details. This is a feature that is not at all peculiar to our state. 

 Leonhardt says, in speaking of quartz porphyries, — "Who that has trav- 

 versed regions of porphyry has not noticed how often each hill and rock 

 shows new peculiarities." This diversity, which is here so marked, and 

 only the most general features of which have been pointed out, render 

 these porphyries very interesting. One can read in the chapters written 

 by Prof. Hitchcock how very widely distributed over our state are the 

 porphyries, and how many mountains and cliffs are made of them. Many 

 more might be described, but their differences are owing only to minor 

 variations of the peculiarities already mentioned. 



Orthoclase PorpJiyry. This rock differs from quartz porphyry in that 

 no quartz is macroscopically visible. Under the microscope, the ground 

 mass is found to consist of the same mixture of minute grains of quartz 

 and feldspar, with which a larger or smaller number of accessories are 

 intermingled. Typical specimens of these rocks are found in Albany, 

 and at Mt. Pleasant, which is just over the boundary of our state in 

 Maine. In Albany, some of these rocks certainly occupy dykes ; and 

 Mt. Pleasant, which is largely composed of this variety of porphyry, is 



