Sienite. 451 



of a more perfect chemical union than any rock resulting from aque- 

 ous agency ever exhibits, unless it be entirely crystalline. 



Under what peculiar circumstances the matter composing porphy- 

 ry was indurated, so as to prevent the greater part of the mass from 

 assuming a crystalline state, it seems to me difficult to conjecture : 

 and, therefore, 1 will not indulge in any speculations upon it. The 

 gradual passage of this rock into sienite, without any apparent change 

 of ingredients, seems to indicate that the peculiarities of porphyry did 

 not result chiefly from the nature of the materials employed in its 

 production, 



17. SIENITE. 



Most writers define classic sienite to be essentially composed of 

 feldspar and hornblende : the first ingredient in the greatest quantity. 

 It is essential also, that it should be connected with the overlying or 

 trap rocks. This definition corresponds but to a small part of the si- 

 enite of Massachusetts. The greater part of it contains quartz as a 

 constant and somewhat abundant ingredient, and mica is very often 

 present. The most of our sienite is the granitic variety, and 

 might perhaps have been described as sienitic granite. Yet in 

 the eastern part of the state it is very intimately associated with 

 greenstone and, therefore, I have thought it best to describe all 

 the varieties of rock, between greenstone and porphyry on one 

 side, and common granite on the other, into whose composition 

 hornblende enters, under the name of sienite. Macculloch's de- 

 finition, which he says " alone rigidly accords with the common 

 definition of sienite," embraces the most important variety of our sie- 

 nite : viz. a granitiform mixture of feldspar, hornblende, and quartz. 

 And this definition I shall take as the type of the rock now to be de- 

 scribed. 



The original specimen to which Werner applied the name sienite 

 came from Sienna in Upper Egypt, whence were obtained many of 

 the well known Egyptian monuments, such as Cleopatra's Needle, 

 Pompey's Pillar, &c. But this rock, it is now well known, is noth- 

 ing but a red granite with black mica and a very small proportion of 

 hornblende; a rock quite different from what Werner supposed. 

 This mistake has occasioned almost endless confusion in geological 

 descriptions. Roziere proposed a remedy. Ascertaining that mount 



