Geology, S/-c. of the Connecticut. 21 



posed to regard this rock as gneiss containing an accidental 

 proportion of hornblende ; and this would have been a satis- 

 factory description of a considerable part of the rock which 

 I have called hornblende slate. But another part appears 

 to be decidedly that species of Werner's primitive trap de- 

 scribed under the name of hornblende slate in Rees Cyclo- 

 pedia, Article Trap — that is, it consists of hornblende, gen- 

 erally fibrous and crystalline, having a very distinctly slaty 

 structure. For localities of this well characterized horn- 

 blende slate I would mention the eastern part of Halifax, 

 Vermont, also New Fane and Belchertown, two miles north 

 of the meeting house on the west side of the road, and in 

 the western part of Tolland and Monson. 



I think however that the largest part of this rock will be 

 found to consist of hornblende, quartz and mica — the latter 

 being usually black and very apt to be confounded with the 

 hornblende, so that perhaps it deserves the name of a granitic 

 aggregate. In some instances, also, this rock contains chlo- 

 rite, and verges towards greenstone slate. It is often 

 strangely intermixed, and alternates with gneiss and mica 

 slate. 



Another portion of this rock has a porphyritic aspect, 

 I use the term porphyritic in this place, not in the usual 

 sense, as denoting a compact ground with imbedded crys- 

 tals, but as a "granite ground, in which some crystals are 

 much larger than the rest." {BakeweWs Geology, p. 28.) 

 The slaty structure of the rock, though less distinct, is not 

 lost : but the imbedded fragments^ or imperfect crystals of 

 quartz or felspar, most frequently the former, give it a 

 porphyritic appearance. These imbedded fragments are 

 frequently granular, while the base is distinctly crystalli- 

 zed. A good example of this variety of rocks occurs in the 

 west part of Chatham and in Shelburne. Sometimes this 

 rock becomes the real sienitic porphyry of authors — its 

 slaty structure being lost. This occurs in Plainfield, in 

 Hawley, a few rods west of the meeting-house, and at the 

 falls in Deerfield river in Shelburne^ 



These porphyritic rocks, however, must be quite different 

 from any thing occurring in Europe by this name, if a re- 

 mark of Brongniart be correct, that " we are not at present 

 able to find a sienite or porphyry which is evidently primi- 

 tive." For we have as much evidence of the primitive char- 



