74 SCIENTIFIC AGRICULTURE. 



gravel, clay, soils, <fec., which constitue a part of its crust 

 " Taken as a whole, the earth is about five times heavier than 

 water, and two and a half times heavier than common rocks." 

 The density of the earth increases from the surface towards 

 the centre. The surface of the earth beneath the ocean, as 

 well as the dry land, is elevated into hills, with plains and 

 Tallies intervening. The mean depth of the ocean is estimated 

 at between two and three miles; from the phenomena of tides, 

 the Atlantic, in its middle part, is supposed to be over nine 

 miles deep. 



DEFINITION OF TERMS. 



Rocks are divided into two great classes, viz: stratified and 

 unstratified. 



Stratification consists of the division of a rock into regular 

 parallel planes or leaves, varying in thickness from that of thin 

 paper, to several yards. Strata are often tortuous and variable 

 in thickness in different parts of the same lamina or layer; 

 "nevertheless, the fundamental idea of stratification, is that of 

 parallelism in the layers." "The term stratum is sometimes 

 employed to designate the whole mass of a rock, while its 

 parallel subdivisions are called beds, or layers." So, also, of 

 sand, clay, gravel, &c. 



The term bed is used to designate a layer or mass of rock 

 more or less irregular, lenticular or wedge shaped, lying 

 between the layers of another rock, such as beds of coal, 

 gypsum or iron. 



Fig. 1. 



- - - - "Without lamina. 



- With waved lamina. 



Finely laminated. 



- Coarsely laminated. 



- - - Obliquely laminated 



- - - - Parallel lamina. 



