10 COMPONENTS OF STRATA. [Oh. It 



on the volcanic or trappean formations, nor on strata containing organic 

 remains. They are hypogene, as " being under" all the rest. 



From what has now been said, the reader will understand that each 

 of the four great classes of rocks may be studied under two distinct 

 points of view ; first, they may be studied simply as mineral masses de- 

 riving their origin from particular causes, and having a certain composi- 

 tion, form, and position in the earth's crust, or other characters both 

 positive and negative, such as the presence or absence of organic re- 

 mains. In the second place, the rocks of each class may be viewed as 

 a grand chronological series of monuments, attesting a succession of 

 events in the former history of the globe and its living inhabitants. 



I shall accordingly proceed to treat of each family of rocks ; first, in 

 reference to those characters which are not chronological, and then in 

 particular relation to the several periods when they were formed. 



CHAPTER II. 



AQUEOUS ROCKS — THEIR COMPOSITION AND FORMS OF STRATIFI- 

 CATION. 



Mineral composition of strata — Arenaceous rocks — Argillaceous — Calcareous- 

 Gypsum — Forms of stratification — Original horizontality — Thinning out — Diag- 

 onal arrangement — Ripple mark. 



In pursuance of the arrangement explained in the last chapter, we shall 

 begin by examining the aqueous or sedimentary rocks, which are for 

 the most part distinctly stratified, and contain fossils. "We may first 

 study them with reference to their mineral composition, external appear- 

 ance, position, mode of origin, organic contents, and other characters 

 which belong to them as aqueous formations, independently of their age, 

 and we may afterwards consider them chronologically or with reference 

 to the successive geological periods when they originated. 



I have already given an outline of the data which led to the belief 

 that the stratified and fossiliferous rocks were originally deposited under 

 water ; but, before entering into a more detailed investigation, it will bo 

 desirable to say something of the ordinary materials of which such 

 strata are composed. These may be said to belong principally to three 

 divisions, the arenaceous, the argillaceous, and the calcareous, which are 

 formed respectively of sand, clay, and carbonate of lime. Of these, the 

 arenaceous, or sandy masses, are chiefly made up of siliceous or flinty 

 grains ; the argillaceous, or clayey, of a mixture of siliceous matter, 

 with a certain proportion, about a fourth in weight, of aluminous earth ; 



