Common Minerals and Rocks
INTRODUCTION.
Minerals and rocks, or the inorganic portions ofthe earth, constitute the proper field or subject-matterof the science of Geology. Now the inorganic earth,like an animal or plant, may be and is studied in threequite distinct ways, giving rise to three great divisionsof geology, which, as will be seen, correspond closelyto the main divisions of Biology.
First, we may study the forces now operating uponand in the earth—the geological agencies—such asthe ocean and atmosphere, rivers, rain and frosts,earthquakes, volcanoes, hot springs, etc., and observethe various effects which they produce. We are concernedhere with the dynamics of the earth; and thisis the great division of dynamical geology, correspondingto physiology among the biological sciences.
Or, second, instead of geological causes, we maystudy more particularly geological effects, observingthe different kinds of rocks and of rock-structure producedby the geological agencies, not only at thepresent time, but also during past ages. This methodof study gives us the important division of structuralgeology, corresponding to anatomy and morphology.
All phenomena present two distinct and oppositeaspects or phases which we call cause and effect; andso in dynamical and structural geology we are reallystudying the opposite sides of essentially the sameclasses of phenomena. In the first division we studythe causes now in operation and observe their effects;and then, guided by the light of the experience thusgained, we turn to the effects produced in the past andseek to refer them to their causes.
These two divisions together constitute what isproperly known as physiography; and they are bothsubordinate to the third great division of geology,—historicalgeology,—which corresponds to embryology.
The great object of the geologist is, by studyingthe geological formations in regular order, from theoldest up to the newest, to work out, in their propersequence, the events which constitute the earth’shistory; and dynamical and structural geology aremerely introductory chapters, the alphabet, as it were,which must be learned before we are prepared to readunderstandingly the grand story of the geologicalrecord.
Our work in this short course will be limited to thefirst two divisions,—i.e., to dynamical and structuralgeology. We will attempt, first, a general sketch ofthe forces now concerned in the formation of rocksand