238 



STRATIFIED AND 'JNSTRATiriED ROCKS. 



unstratified, therefore, apply to tL« same class of rocks — the first indica- 

 ting their origin, the second their structure in the small, the third their 

 structure in the large, as distinguished from that of the rocks which occur 

 in beds. 



The following diagram exhibits the general appearance of tlje strati 

 fied rocks as they are found to occur in contact with unstratified masses 

 in various parts of the globe : — 



J) 



A represents an unstratified mountain mass or other similar rock rising 

 up through the stratified deposits. The bending up of the edges of the 

 latter indicates that after the beds were deposited in a nearly level posi- 

 tion, the mass A was intruded or forced up through them, carrying the 

 broken edges of the beds along with it. 



B shows the more quiet way in which veins or dykes of unstratified 

 green-stone, or trap, or lava, cut through the beds without materially 

 displacing them — as if when in a fluid state it had risen up and filled a 

 previously existing crack or chasm. In Devonshire, in the North of 

 Scotland, a'nd in Ireland, the granite rises i« many places exactly as is 

 shown at A, and nearly all our coal fields exhibit in their whin dyke.s 

 numerous illustrations of what is shown at B. 



C and D exhibit the manner in which the strata overlie one another 

 in nearly a horizontal position — 1, 2, 3, indicating difl!erent kinds of rock, 

 — as a lime-stone, a sand-stone, and a clay — which again are subdivided 

 into beds or thinner layers, by the partings exhibited in the wood-cut. 



The stratified rocks lie sometimes nearly level or horizontal over large 

 tracts of country — as in the above diagram, — sometimes they are more 

 or less inclined or appear to dip in one and to rise in the opposite direc- 

 tion — as if a surface, formerly level, had been pushed down at the one 

 end and raised up at the other, — and sometimes they seem to rest entire- 

 ly upon their edges. Upon the mode in which they thus lie, the unifor- 

 mity of the soil, in a district where it reposes immediately on the rocks 

 from which it is derived, is materially dependent. In the following dia- 

 gram the surface from A to S represents a tract of country in which the 



rocks have in different parts these different degrees of inclination, at A 

 vertical, at B more inclined, and from C to E nearly horizontal. Now, 

 it is obvious that if the outer surface of these several rocks crumble and 

 form a soil which rests where it is produced — then the quality of the soil 

 on every spot will be determined by the nature of the rock beneath. 

 Hence, in proc^eeding from E over the comparatively level strata, we 

 shall find the soil pretty uniform in quality till we come to the edge of 



