PART I. CHAPTER I. 15 



Classification of Rocks Aqueous Rocks. 



In order to classify the various rocks which compose the 

 earth's crust, it is found most convenient to refer, in the first 

 place, to their origin, and in the second to their age. I shall 

 therefore begin by endeavouring briefly to explain to the student 

 how all rocks may be divided into four great classes by refer- 

 ence to their different origin, or, in other words, by reference to 

 the different circumstances and causes by which they have been 

 produced. 



The first two divisions, which will at once be understood as 

 natural, are the aqueous and volcanic, or the products of watery 

 and those of igneous action. 



Aqueous rocks. The aqueous rocks, sometimes called the 

 sedimentary, or fossiliferous, cover a larger part of the earth's 

 surface than any others. These rocks are stratified, or divided 

 into distinct layers, or strata. The term stratum means simply 

 a bed, or any thing spread out or strewed over a given surface ; 

 and we infer that these strata have been generally spread out by 

 the action of water, from what we daily see taking place near 

 the mouths of rivers, or on the land during temporary inunda- 

 tions. For, whenever a running stream, charged with mud or 

 sand, has its velocity checked, as when it enters a lake or sea, 

 or overflows a plain, the sediment, previously held in suspension 

 by the motion of the water, sinks, by its own gravity, to the 

 bottom. In this manner layers of mud and sand are thrown 

 down one upon another. 



If we drain a lake which has been fed by a small stream, we 

 frequently find at the bottom a series of deposits, disposed with 

 considerable regularity, one above the other; the uppermost, 

 perhaps, may be a stratum of peat, next below a more dense and 

 solid variety of the same material ; still lower a bed of laminated 

 shell-marl, alternating with peat or sand, and. then other beds of 

 marl, divided by layers of clay. Now if a second pit be sunk 

 through the same continuous lacustrine formation, at some dis- 

 tance from the first, we commonly meet with nearly th^. same 

 series of beds, yet with slight variations ; some, for example, of 

 the layers of sand, clay, or marl, may be wanting, one or more 

 of them having thinned out and given place to others, or some- 

 times one of the masses first examined is observed to increase in 

 thickness to the exclusion of other beds. 



The term "formation" which I have used in the above 

 explanation, expresses in geology any assemblage of rocks which 

 have some character in common, whether of origin, age, or 

 / composition. Thus we speak of stratified and unstratified, fresh- 

 water and marine, aqueous and volcanic, ancient and modern, 

 metalliferous and non-metalliferous formations. 



