LITHOLOGY, 1 39 



of lithological literature. Every new region, however, furnishes new 

 variations on old facts, and thus helps to strengthen, and sometimes to 

 build up. 



Before proceeding to the description of particular species of rocks, a 

 few remarks of a general character, applicable to the rocks of our 

 region, may make more simple the method of arrangement adopted in 

 this treatise. 



A hundred minerals have been described as occurring in our state ; of 

 these, more than half are now classed out as of no importance to lithol- 

 ogy, and the diversity in our rocks is produced by the various combina- 

 tions of the remainder. At times we have a simple aggregate of one 

 mineral, and, again, a most complex mixture. Yet it is not like the for- 

 mation of words, without number, by combinations of the letters of the 

 alphabet. The subject obtains interest from the fact that rocks resultant 

 from the combinations of minerals are limited in number and in kind by 

 certain chemical considerations. Now, the mere determination of the 

 mineral ingredients of a rock has little besides an economic importance. 

 The subject, as a science, has its chief interest in the study of the condi- 

 tions under which our earth has become thus covered with such very 

 diverse accumulations of material; how the particular minerals became 

 combined in such ways ; and how they obtained their present form and 

 condition. Bound thus to a central idea, the study of the mineral com- 

 position becomes interesting; and this central idea should give the basis 

 for classification and arrangement of the material for study. 



The following is the general mode of origin of diversity in rocks: The 

 earth was once in a condition of igneous fluidity, and while in this con- 

 dition, with the particles of matter freely movable, the various materials 

 that composed the earth's outer zone would enter into their most stable 

 combinations, and would form one immense, homogeneous mass; and 

 thus the first crust of the earth may be supposed to have been quite uni- 

 form in character and composition, with only the variations induced by 

 gravity, which would draw heavier materials to a lower level. But the 

 conditions of chemical stability, in a state of igneous fusion, are quite 

 different from those in the cold. Not merely are certain elements, with 

 strong affinities for one another, but which in the heat are separated from 

 one another by different degrees of volatility, brought to act upon one 



