Origin ofSienite. 



463 



sketch will give an idea of the situation of the conglomerated sienite 

 there. It is the northern point of a range extending through Hatfield 

 and Northampton southerly : and it here abuts against a limited de- 

 posit of hornblende slate, whose strata run nearly N. E. and S. W, 

 as «, and stand nearly perpendicular to the horizon. 



This slate is succeeded on the west by mica slate b, b, b, which in- 



deed, sometimes alternates with the hornblende slate. On the eastern 

 side d, d, d, the sienite and the slate are covered by the diluvium and 

 new red sandstone of the valley of the Connecticut. The sienite and 

 hornblende slate are elevated not more than 100 or 200 feet above 

 the general level of that valley : but directly west of these rocks the 

 mica slate rises more rapidly into ridges of much greater elevation, 

 forming the eastern margin of the broad range of Hoosac mountain. 



Now I have been led by an examination of the spot just described, 

 (whose length is 3 or 4 miles and breadth less than one,) to conclude 

 that the sienite was formed by the melting down of the hornblende 

 slate. I infer this chiefly from the fact that this rock, as has been 

 described, contains nodules of this slate, appearing as if round- 

 ed not by mechanical attrition but by heat. Why it should happen 

 that the fusion of this slate should give rise to the production of feld- 

 spar, we may not be able fully to understand. And yet, if we suppose 

 the hornblende slate to be composed of hornblende, mica and quartz, 

 as it sometimes is in Whately, or that it alternates with mica slate, as 

 it does there, we shall have in the materials melted down, all the es- 

 sential ingredients of feldspar, viz. silex, alumina, and potassa. A 

 certain degree of heat may be all that is necessary to enable these el- 

 ements to enter into the new combination that is necessary to the pro- 

 duction of feldspar. At any rate, I think I am not mistaken in the 

 fact, that as the imbedded nodules in the slate approach more nearly 



