INTRODUCTION. 
5 
earth are not uniformly continuous in all direc- 
tions over large superficial areas. In one district, 
we trace the course of crystalline and granitic 
locks; in another, we find mountains of slate; 
in a third, alternating strata of sandstone, shale, 
nnd limestone ; in a fourth, beds of conglomerate 
rock ; in a fifth, strata of marl and clay ; in a 
sixth, gravel, loose sand, and silt. The subor- 
dinate mineral contents of these various forma- 
tions are also diflferent ; in the more ancient, 
are veins of gold and silver, tin, copper, lead 
and zinc ; in another series, we find beds of 
coal; in others, salt and gypsum; many are 
composed of freestone, fit for the purposes of 
architecture ; or of limestone, useful both for 
building and cement; others of clay, conver- 
tible by fire into materials for building, and 
pottery : iu almost all w'e find that most im- 
portant of mineral productions, iron. 
Again, it we look to the great phenomena of 
physical geography, the grand distributions of 
the solids and fluids of the globe; the disposition 
ot continents and islands above and amidst the 
waters ; the depth and extent of seas, and lakes, 
nnd rivers ; the elevation of hills and mountains ; 
the extension of plains ; and the excavation, de- 
pression, and fractures of valleys ; we find them 
all originating in causes which it is the province 
af Geology to investigate. 
A more minute examination traces the pro- 
