lyS MINERALOGY AND LITHOLOGY. 



Porphyritic felsites are sub-divided into three species. Qtiartr: por- 

 phyry consists of a felsitic mass of quartz and orthoclase, in wliich ma- 

 croscopic crystals or grains of both these ingredients are developed. 

 Orthoclase potphyry consists of the same ground mass in which ortho- 

 clase alone is porphyritically developed. Qjiartz-frce-ortJtoclase porphyry 

 contains quartz neither porphyritically developed, nor in the ground mass. 

 The last division, though represented, is of no practical importance in 

 New Hampshire. 



Quart:: Porphyry. Almost all the porphyries which occur in the state 

 belong to this division. The ground mass is a felsite in which the pro- 

 portion between the quartz and feldspar is variable, and in which mag- 

 netite, augite, hornblende, biotite, chlorite, hematite, apatite, and some 

 other minerals may be present as accessories. Of the porphyritic min- 

 erals, the feldspar has usually a crystalline outline, and sometimes the 

 quartz also. The greatest diversity in the appearance of the rocks pre- 

 sents itself. Some are black, some are gray, and some are red, and they 

 may have a ground mass of any one of these three colors, and porphyritic 

 crystals developed in it of any other of the colors; and so the variations 

 become very numerous. This rock forms small dykes; and, on the other 

 hand, immense mountains like Mt. Kearsarge are largely composed of it. 

 Our porphyries are commonly massive rocks with no signs of structure, 

 except in those cases where they possess a schistose nature that has 

 been induced in them by external agencies. Their geological relation- 

 ships, though sometimes evident, are more often obscure. 



In the north-east part of Waterville a beautiful black porphyry is abun- 

 dant, which, on account of the fine opalescence of its clear grains of feld- 

 spar, has been supposed to be a dolerite or labradorite rock, but the thin 

 sections show that only orthoclase feldspar is present. The ground mass 

 is very feldspathic, and contains some augite, chlorite, magnetite, apatite, 

 and scales of hematite. The feldspar is clear, but is filled with minute 

 fissures and microlites, reflections from which cause the irridescence. 

 The rock is a beautiful one, and it would be much admired if cut and 

 polished. It was analyzed by Mr. Pease, in the Shefifield laboratory, with 

 the following result : 



Silica, 63.63 



Alumina, 1742 



