GEOGNOSY AND GEOLOGY. 101 
mixture of serpentine and compact limestone. The limestone is often 
separated as marble, and veins colored by different substances give it a 
very beautiful appearance ; verd antique marble is here included. 
Order 7. Amphibolic Rocks. 
Minerals of amphibolic character, as grammatite, actinolite, asbestos, 
anthophyllite, hornblende, &c., form the predominant ingredients in rocks 
of this order. It is hornblende, however, either pure or in combination 
with other bodies, which forms the mass of the rock. 
Hornblende rock is nothing more than hornblende in a pure state, or with 
a few unimportant additions. ‘T'wo kinds of this rock are distinguished : 
hornblende-rock proper, and hornblende-slate. The former is crystalline 
granular, and less hard than tough. It is therefore difficult to break or 
blast, and forms a considerable impediment in the way of mining or 
excavating. Hornblende-slate, which consists of scaly hornblende, possesses 
a rather imperfect stratification, with the same greenish-black color as the 
last. Actinolite and anthophyllite occur in both varieties, as unessential 
ingredients. Other minerals also occur, and cause the transition and 
hornblende flagging-stone, hornblende gneiss, syenite, diorite, &c. 
Diorite (greenstone). This rock was formerly considered to stand in 
such a relation to diabase, as to be entitled to consideration only as a 
variety of the latter. The more careful investigations of Hausmann have, 
however, shown that they are essentially different. While the former 
contains a pyroxene mineral, hypersthene, as the predominating constituent, 
in combination with a feldspathic, as labradorite, or albite, and chlorite , 
the latter (diorite) is a more or less distinct mixture of an amphiboloid 
substance, hornblende, with albite. It is, in composition, nearly allied to 
syenite. The hornblende imparts to it a dark greenish-black color. The 
difference in the manner in which the combination of the constituents 
takes place, permits a distinction into: granular diorite, whose particles are 
crystalline granular; globular diorite, in which its granules are enveloped 
by concentric coats of compact feldspar, or a variety of hornblende; 
porphyritie diorite, where albite is interspersed in the mass; and compact 
diorite (aphanite) consisting of a very intimate mixture of the above- 
mentioned minerals. Both the diorite and hornblende rocks are capable 
of withstanding the action of the atmosphere to a very high degree. The 
feldspathic matter weathers first, and gives rise to a very rough surface, 
well calculated for the abode of various cryptogamous plants. Umbilicarice. 
for instance, occur in such localities in great perfection and profusion. 
Order 8. Calcareous Rocks (Limestones). 
The carbonate of lime of mineralogy, either pure or combined with other 
substances, is an exceedingly important constituent of the earth’s crust. 
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