72 
AMERICAN GEOLOGY. 
SIENITE. 
§ 60. Sienite is a granite in which hornblende takes the 
place of mica. This is more abundant than mica. The latter 
may be present in small quantities, and it may be absent. 
This rock is to be distinguished from hornblende rock, how¬ 
ever, by the absence of lamination. Its structure is granitic in 
the true sense of the word. Taking granite as the type of 
sienite, it may, like the former, be divided into varieties, either 
by the absence of one of its elements, or by the form or shape 
which one of its elements has assumed. 
§ 61. Feldspar and hornblende. By reference to the compo¬ 
sition of greenstone, it will be seen that this is also composed 
of the same substances, but the particles in greenstone are 
minute, and unless it is porphyritic, it appears homogeneous. 
But this variety is made up of distinct particles of hornblende 
and feldspar. There is, however, a gradation of this variety 
into greenstone, or an approach to greenstone. This fact may 
be observed at Nahant in Massachusetts. 
Fig. 4. 
a Limestone, b Granite. 
Sienite and granite are often associated with limestone. 
Orange and St. Lawrence counties furnish many instances: fig. 
4 is an example of the relation of the two rocks at Fowler, St. 
Lawrence county. The figure shows the structure of granite. 
That limestone is strictly a pyrocrystalline or eruptive rock 
at all the localities which I have cited, appears from the fact 
that where it is in contact with the Potsdam sandstone the 
latter rock is vitrified. It has lost its granular structure, and 
