78 



HORNBLENDE ROCK. 



into serpentine and soapstone. These slates are 

 often confounded. 



We now come to the subordinate rocks among the 

 primary, the first of which, in our division, is, 



1. Hornblende Rock, 



In addition to the description already given of this 

 mineral, we may remark, that when it forms the 

 principal part of rocks, the colour is commonly a 

 greenish black. It occurs in beds in granite, gneiss, 

 and mica slate, and occasionally in argillaceous 

 slate. Bakeweil states that it passes into serpen- 

 tine by an increase of magnesia, which forms one 

 of the constituent parts of hornblende. When 

 hornblende and feldspar are coarscty blended, they 

 form greenstone ; and when so incorporated as to 

 form an apparently homogeneous mass, they form 

 basalt, or a trap rock which has all the characters 

 of basalt. Professor Hitchcock remarks, that every 

 deposite of hornblende slate that he has examined 

 in Massachusetts is associated either with gneiss, 

 talcose slate, mica slate, or quartz rock ; and that he 

 regards them all as belonging to the same geologi- 

 cal epochs, and produced by essentially the same 

 causes. This rock has a tendency to divide into 

 rhomboidal masses, and its layers frequently ex- 

 hibit numerous and complicated contortions. It 

 contains but few interesting minerals, among which 

 are garnets^ epidote, and sphene. 



Dr. MacCulloch remarks, that "the origin of 

 nornblende slate from clay slate is completely es- 

 tablished by the occurrence in Shetland of a mass 

 of the latter substance, alternating with gneiss and 

 approximating to granite. Here these portions 

 •which come into contact with the latter become 

 first silicious schist, and ultimately hornblende 

 schist : so that the very same bed which is an inter- 

 *amination of gneiss and clay slate in one part, is 

 in another the usual alternation of gneiss and horn- 



