2 INTRODUCTION. 
cessities of mankind have induced them to explore, 
satisfactorily demonstrate the wisdom which has been 
displayed in the arrangement of materials requisite for 
the use of men and animals. 
The first layer is frequently a rich, black mould, 
formed almost wholly of animal and vegetable remains. 
This yields sustenance to the vegetable productions; 
and thereby becomes the actual, though not the imme- 
diate, support of the whole animal creation. Beneath 
this is often found a thick bed of clay, that furnishes to 
man a substance of which to make bricks, tiles, various 
kinds of pottery, and innumerable other articles for the 
comfort of social life. Next are deposited vast beds of 
gravel, that are of use in numerous points of view. 
Underneath this are the infinitely varying strata of sand- 
stone, limestone, &c. which not only serve for the con- 
struction of buildings, and for other important purposes, 
but also frequently surround mines which contain the 
valuable metals. Beneath a slaty stratum are usually 
discovered those immense beds of coal so requisite for 
the comfort, and, in some situations, even for the exist- 
ence of man. 
These strata, it is true, are not always found together, 
nor are they always discovered in the same order ; but 
the statement will suffice to show the general nature of 
their arrangement. 
3. Minerals are sometimes observed in detached 
masses of various size, and situated at various depths in 
the earth (Fig. 3). 
4. They are also found in a kind of natural clefts 
which cross the regular mineral beds or strata in differ- 
ent directions (Fig. 1, a, Fig. 4, b). When these 
