LITIIOLOGY. 187 



regarded as an eruptive mass. The rocks are alike in both places, and 

 one description will apply to both. They are either gray or very light 

 red. The ground mass is usually very fine in its texture, but it is often 

 porous and rough, and often very much resembles that of some modern 

 trachytes. Were it not that the feldspar is opaque orthoclase instead 

 of clear sanidin, one would immediately think of trachyte upon examin- 

 ing these rocks. At some points an Mt. Pleasant the rocks are cracked 

 up into angular blocks and fragments, as are many modern eruptive 

 rocks; and in other places large crystalline grains of hornblende are 

 prominent, and cause other kinds of trachytes to be recalled. Trachyte 

 is a newly erupted rock, which, in addition to its glassy feldspar, con- 

 tains noncrystalline substances, — nephelin, leucite, and the like; and 

 though in special varieties any or all of its distinctive features or char- 

 acteristic minerals may be wanting, the rocks as a class are very well 

 distinguished, and no one would wish to confound such old rocks as 

 these that we are considering with them. Still, I think these orthoclase 

 porphyries from Albany and Mt. Pleasant are very interesting, since, by 

 their very close macroscopic resemblance to certain trachytes, they show 

 the close relationship which exists between these rocks and their younger 

 kindred. Under the microscope no peculiarities of note are developed. 

 Hornblende is found to be a common constituent; the feldspar is usually 

 troubled by impurities and decomposition, but in some specimens it is 

 quite fresh, though never glassy. On the summit of the mountain the 

 ground mass is reduced to a minimum in quantity, and the rock ap- 

 proaches sienite in appearance and composition. 



Quartz-Free OrtJioclase Porphyry. This rock has the same structure 

 as the other porphyries, but possesses quartz neither among the macro- 

 scopic crystals nor in the ground mass. A rock of limited distribution 

 anywhere, it is of no practical importance in New Hampshire, for it oc- 

 curs as a mere exceptional variety in one or two places among the quartz 

 porphyries. For example : a specimen of red porphyry from Waterville 

 and another from Albany are remarkable because no quartz can be de- 

 tected in them, while at the same time a triclinic feldspar, conspicuous 

 in polarized light by its bands of color, appears as an ingredient, and by 

 its presence suggests the more basic nature of the rock. 



In these varieties of porphyry in which large crystals of quartz do not 



