• LITIIOLOGY. 209 



carbonic acid were generated as suggested, the conditions of heat and 

 pressure were such as to bring about only a partial reaction; and the con- 

 ditions under which granitic rocks have been recrystallized, as indicated 

 by other circumstances, are just about such as would account for the 

 associations observed in this rock. 



Another sienite, which in most respects is exactly like this, occupies a 

 dyke in Sandwich. Calcite is absent, and with this absence the quantity 

 of carbonic acid decreases. In many cavities in the quartz, however, 

 liquid carbonic acid is found, but in the larger number only water and 

 the customary crystals of salt. These two rocks are our only ones in 

 which I have found this interesting enclosure of liquefied gas, but they 

 are remarkable examples. 



On Mill mountain, in Stark, a dark-colored sienite occurs. Macroscop- 

 ically, it appears as if wholly composed of orthoclase ; but in thin sec- 

 tions considerable hornblende is found in small fragments, which are 

 quite thickly distributed : if the rock were light-colored, as are the 

 ones thus far described, the hornblende would be prominent enough. 

 The feldspar is of that clear grayish waxy variety (adularia) which is so 

 prominent in a group of granites which have been described, and to 

 those granites this sienite is related. It contains also plagioclase, apa- 

 tite, sphene, and epidote, besides a little microscopic quartz. 



A very different kind of sienite is abundant about the town of Albany. 

 This is a very fine-grained feldspathic rock which is found in small 

 dykes. It is red in color, and full of minute black specks. When thin 

 sections are cut, these specks are found to be of a very deep-colored 

 hornblende with some little crystals of magnetite, while the mass of the 

 rock is composed of small but well defined crystals of orthoclase, in the 

 angles between which a little quartz is found. These rocks have com- 

 monly been called felsites ; but in no respect do they differ from ordi- 

 nary sienite, save in the fineness of their texture. On Sabba-Day brook 

 there are favorable localities to see these rocks in place. 



GNEISS. 



Gneiss is in composition exactly like granite, being composed essen- 

 tially of quartz and orthoclase as constant constituents, and muscovite, 



biotite, hornblende, chlorite, etc., as essential but variable constituents, 

 VOL. IV. 27 



