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. Bakewell 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 coarsely 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- 
tabhshed 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- 
lamination of gneiss and clay slate in one part, is 
in another the usual alternation of gneiss and horn- 
