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 
rocks ; in another, we find mountains of slate ; 
in a third, alternating strata of sandstone, shale, 
and 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 different; 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, convertible 
by fire into materials for building, and pottery : 
in almost all we find that most important of 
mineral productions, iron. 
Again, if we look to the great phenomena of 
physical geography, the grand distributions of 
the solids and fluids of the globe; the disposition 
of continents and islands above and amidst the 
waters ; the depth and extent of seas, and lakes, 
and 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 
of Geology to investigate. 
A more minute examination traces the pxO- 
