Ranges of Hornblende Slate. 



379 



stone, where the slaty and stratified tendency begins to be lost in the 

 trappose or columnar : next we strike the peculiar variety, already 

 described, as containing white compact feldspar : finally succeeds 

 mica slate. In describing sienite I shall refer again to this spot. 



The strip of hornblende slate marked along the eastern margin of 

 the gneiss range in the west part of Hampden county, and east part 

 of Berkshire, is so intermingled with the gneiss on one side, and 

 with the mica slate and talcose slate on the other, that it is no easy 

 matter to fix upon its true position or extent. I have exhibited it in 

 those places where I found it most abundant. But viewing it rather 

 as a variety of gneiss, I do not regard its exact situation or extent on 

 the map of any great importance. Traces of this same rock may be 

 seen all along the eastern margin of the talcose slate ; and in Plain- 

 field and Hawley, it is not uncommon to find examples of pure horn- 

 blende slate: though usually the hornblende forms the least abundant 

 ingredient, and ought perhaps to be considered merely as crystallized 

 hornblende disseminated through talcose slate. 



Some of the hornblende rock in Granville and Tolland is lamin- 

 ated, and the size of the laminae is often gigantic. In the latter 

 place, half a mile east of the meeting house, I have seen imperfectly 

 prismatic masses not less than a foot in length and one or two inches 

 wide. I apprehend that this hornblende slate in these towns, is con- 

 nected with the extensive ranges of that rock which one crosses in 

 passing from Lee to Becket. But I am so doubtful about the true 

 situation of this rock along the western part of the gneiss range in 

 Berkshire county, that I have forebore to represent it on the map. 

 In the east part of Lee and west part of Becket, it is associated 

 Avith augitic gneiss : and both these rocks are obviously varieties of 

 gneiss. 



In respect to the gneiss, hornblende slate, mica slate, and talcose 

 slate, represented on the map as occupying a considerable part of the 

 Hoosac range of mountains, I take this opportunity to remark, that 

 whoever expects in passing transversely or longitudinally over this 

 region, to find as sudden and decided changes in the rocks as are 

 shown on the map, will be disappointed. On a map the change must 

 be represented as sudden : but I have often travelled for miles in that 

 region, in great uncertainty what rock prevailed. In such cases, 

 one can give the line between different formations only by approxi- 

 mation. 



