4 MINERALOGY AND LITHOLOGY. 
composition, mode of origin, and instructive peculiarities of our rocks 
and minerals. 
A rock is a mineral aggregate. It is a mass that is composed either of 
one mineral, or of a mixture of several. Hence, in studying the materials 
of the earth’s crust, we must begin with mineralogy ; and when we are 
familiar with the simple minerals, we can then study their aggregations, 
which form simple or complex rocks. In this work, therefore, the min- 
erals that have been found in the state will first be enumerated, and their 
noticeable peculiarities will be pointed out. The second part will be 
devoted to a description of the rocks. 
Minerals are often attractive and beautiful as specimens. Natural 
crystals and gems are admired by all. Rocks are generally admired as 
forming masses; but rocks are also beautiful when we study them with 
searching care,—when their minutest structure is brought into view by 
the aid of the microscope. Moreover, most instructive results are obtained 
by this method of study. Other portions of this geological report have 
treated of the age of our minerals and rocks, of their distribution and 
relations to one another, and of the structure of the country which results 
from the method of their arrangement. In this part, the results of labo- 
ratory work are given; and the methods of physical and chemical miner- 
alogy are introduced as a supplement to the work in the field. Micro- 
scopic work has been made prominent, since by this method of study 
such weighty results have been obtained by foreign laborers, that is has 
now become indispensable in the prosecution of geological work. The 
author’s aim has been to apply the newer methods of study to our old 
rocks; to try to show the value of those methods, and how many inter- 
esting things can be observed with their aid. He hopes that the many 
observations new to our section of country, and the variations here fur- 
nished on the observations made elsewhere, will be considered of value. 
He wishes to apologize for the incompleteness of the work, and the injus- 
tice done a grand series of rocks. Where the labors of a lifetime could 
not exhaust the observations that might be made, the work of a very 
short time, on limited material, can do but little more than draw the 
attention of students to this field of study. 
The author does not wish to enter on this work without paying his 
tribute to the German lithologists who have developed the methods that 
