132 SIMILARITY OF CONTIGUOUS .DEPOSITS. 



comprising chlorite and steatite ; 7, augite ; 8, serpentine ; 

 9, iron. Other minerals, which either form rocks of small 

 extent, or enter so largely into their composition as to 

 modify their character, are the following : Sulphate of lime, 

 diallage, chloride of sodium, coal, bitumen, garnet, schorl, 

 staurotide, epidote, olivine, and pyrites. A few of these 

 minerals exist in masses so large as to be denominated 

 rocks ; as quartz, carbonate of lime, sulphate of lime, salt, 

 coal, and pyrites ; but in general some two, three, or four are 

 united to constitute a rock, as mica, quartz, and feldspar, 

 to produce granite. 



The following general remarks descriptive of some of the 

 peculiarities of rocks, both singly and in groups, may be 

 advantageously borne in recollection : 



EXTENT or THE OLDEE FORMATIONS. The oldest deposits 

 are the most extensive, both as regards area and thickness. 

 In the older and more crystalline rocks we find convincing 

 evidence of the action of fire: as we proceed upwards, 

 the traces of this agency become more indistinct, until, in 

 the newer deposits, with the exception of local sites of 

 extinct volcanic action, we lose all vestiges of its influence. 



DEBITED CHAEACTEE or THE SEDIMENTARY FORMATIONS. 

 The sedimentary deposits are produced by the degradation 

 of rocks older than themselves : the sandstones of the coal 

 are extremely micaceous, owing to the mica contained in 

 the primary strata from which they were formed ; the new red 

 sandstone was produced from the detritus of rocks of like 

 metamorphic character, and owes its colour to the iron, mica, 

 or hornblende which the older deposits contained. The dis- 

 integration of rocks of the primary class is supposed to 

 have yielded many of the argillaceous deposits and beds 

 of clay appertaining to later formations, whist the chalkj in 

 like manner, has furnished materials for the formation of the 

 tertiary strata. 



SIMILAEITT OP CONTIGUOUS DEPOSITS. It has been 

 remarked, that a character of transition prevails among 

 many contiguous rocks, that the lower members of one 

 formation resemble the upper deposits of the series below. 

 Thus the inferior beds of the lower greensand frequently 

 oifer a striking resemblance to the superior strata of the 

 wealden ; while the lower members of the oolite, and the 





