Minerals in Hornblende»Slate. 



381 



Mineral Contents. 



So far as it has been examined, no rock in the state appears to be 

 so barren of interesting minerals as this. Garnets are perhaps the 

 most common, and generally they are of a blood red colour — probably 

 in some cases the pyrope. In Rowe, epidote occurs in this rock in 

 a state of such purity that it deserves to be mentioned. In Middlefield 

 and Chester, sphene has been observed in it. In its cavities also, not 

 unfrequently, as in Charlemont and Whately, I have noticed tolera- 

 bly distinct crystals of feldspar. The immediate gangue of the 

 plumbago mine at Sturbridge is hornblende and feldspar ; and the 

 former minerals constitute the gangue, to a considerable extent at 

 least, of the arsenical cobalt of Chatham, Ct. Red oxide of titanium 

 I have also found in hornblende in Leyden. Other minerals will un- 

 doubtedly be discovered in this rock, upon farther examination. 

 Hitherto mineralogists have paid very little attention to the geological 

 situation of localities in our country. 



Theoretical Considerations. 



It is easy to apply to hornblende slate the theory which imputes to 

 the primary rocks an origin partly aqueous and partly igneous. For 

 it is a very fusible rock, and may hence easily be conceived to have 

 been sufficiently heated to enable it to assume the crystalline aspect, 

 which it almost always exhibits. But from what rock did the horn- 

 blende slate originate ? The researches of Dr. Macculloch appear to 

 have thrown a gleam of light upon this difficult question. "As far 

 as a single fact can prove such a case," says he, " the origin of horn- 

 blende schist from clay slate is completely established by the occur- 

 rence in Shetland, of a mass of the latter substance alternating with 

 gneiss and approximating to granite. Here those portions which 

 come into contact with the latter, become first, siliceous schist, and ul- 

 timately, hornblende schist ; so that the very same bed which is an 

 interlamination of gneiss and clay slate in one part, is in another, the 

 usual alternation of gneiss and hornblende schist."* In another place 

 .he says, " it would appear, that the fusion of clay slate, whether pri- 

 mary or secondary, is, under various circumstances, capable of gene- 

 rating, either the common trap rocks, or the hornblende schists : nor 

 is it perhaps difficult to explain, by a more gradual cooling, and con- 



* System of Geology, vol. 1. p. 210. 



