Minerals in Sienite. 461 



newer stratified rocks. Or if we suppose it erupted at a different 

 epoch from the granite, certain causes always forced it upwards be- 

 tween the granite and the newer rocks. Or if we suppose it to 

 have resulted from the melting down of the stratified rocks, then per- 

 haps their more or less perfect fusion produced the difference which 

 we find between granite and sienite. But more of this last sugges- 

 tion in the sequel. 



Mineral Contents. 



The limits between sienite and granite, as well as between sienite 

 and greenstone, are so vague and unsettled, that it is not always easy 

 to ascertain to which of these rocks minerals described by different 

 writers belong ; since in such a case geologists will be apt to give dif- 

 ferent names to the same rock. Genuine sienite, I mean that which 

 is best characterised, is in Massachusetts rather barren .of simple min- 

 erals : not quite so much so, however, as porphyry. By far the most 

 common mineral is epidote ; whose characters and mode of occur- 

 rence I have pointed out. The sienite used for architectural purposes 

 from the vicinity of Boston, contains less of this mineral than any oth- 

 er variety in the state. 



There is an interesting variety in the feldspar of this rock. In 

 Beverly a few years since, a considerable quantity of green feldspar 

 was obtained from a rock near the center of the place. The bronze 

 coloured feldspar of Cape Ann has been already described, and that 

 which is of a lilac colour in Hingham. In Charlestown a variety oc- 

 curs in which the prisms exhibit stripes of various colors, and some 

 have proposed for it the name taenite, on account of its resemblance 

 to a ribbon. On Holyoke and Tom I have described a variety of trap 

 in which this mineral presents a similar appearance. According 

 to Professor Webster, hypersthene occurs in the sienite of Hingham 

 along with hornblende. Amethyst is said to exist in the same rock 

 in that place. 



In the sienite of Beverly, fluate of lime and zircon have been found ; 

 and the former mineral in the rolled masses of this rock in Seekonk. 

 In that of Charlestown Prof. Nuttall discovered arragonite. Preh- 

 nite, according to Prof. Webster, of superior excellence, is found there 

 in the sienite : and that gentleman has recently informed me, that he 

 has discovered in the same rock very superior specimens of chabasiu 

 and laumonite. 



The veins of quartz abounding in delicate crystals in the sienite 



