PART I. CHAPTER II. 25 



Division of the Work Mineral Composition of Stratified Rocks. 



I shall accordingly divide this work into two parts, in refer- 

 ence to these two modes of considering each family of rocks. In 

 the first part, the characters of the aqueous, volcanic, plutonic, 

 and metamorphic rocks will be described, without reference to 

 their ages, or the periods when they were formed. In the second, 

 their different ages will be considered, and I shall endeavour to 

 explain the rules according to which the chronology of rocks in 

 each of the four classes may be determined. 



CHAPTER II. 



AQUEOUS HOCKS THEIR COMPOSITION AND FOEMS OP STRATIFICATION. 



Mineral composition of strata Arenaceous rocks Argillaceous Calcareous 

 Gypsum Forms of stratification Original horizontality thinning out Diagonal 

 arrangement Ripple mark. 



FIRST, then, in pursuance of the arrangement explained in 

 the last chapter, we have to examine the aqueous or sedimentary 

 rocks, which are, for the most part, distinctly stratified, and con- 

 tain fossils. We are to consider them with reference to their 

 mineral composition, external appearance, position, mode of ori- 

 gin, and other characters which belong to them as aqueous 

 formations, without reference to their age, or the various geolo- 

 gical periods when they may have originated. 



I have already given an outline of the data which lead to the 

 belief that the stratified and fossiliferous rocks were originally 

 deposited under water ; but, before entering into a more detailed 

 investigation, it will be 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; 5 and, lastly, the calcareous rocks or limestones consist of 

 carbonic acid and lime. 



Arenaceous or siliceous rocks. To speak first of the sandy 

 division : beds of loose sand are frequently met with, of which 

 the grains consist entirely of silex, which term comprehends all 

 purely siliceous minerals, as quartz and common flint. Quartz 

 is silex in its purest form ; flint usually contains some admixture 

 of alumine and oxide of iron. The siliceous grains in sand and 

 c 



