48 



SUPERPOSITION OF ROCKS. 



evidently, formed after the rocks on which they rest. Even were 

 we to admit the subsequent fusion of granite, it obviously, preexist- 

 ed, in another form, as a substratum of the upper rocks, as these 

 must always have had a foundation. It has been before observed, 

 that those rocks which contain different species of organic remains, 

 separated by strata in which no such remains occur, must have been 

 formed, in succession, over each other, and probably at very distant 

 intervals of time. This inference appears conclusive, nor can it be 

 invalidated, by the crystalline arrangement and cleavage of some of 

 those rocks.* 



Rocks, of the primary class, frequently cover each other in an or- 

 der, which, viewed on a grand scale, may be said to be conforma- 

 ble ; but, the different rocks in each class are generally of such vast 

 and irregular thickness, that their order of succession is often, not 

 easy to trace : besides, some of these rocks pass, by a change of 

 structure, into each other, and their hne of junction or separation 

 can, seldom, be observed. Viewed, however, as composing moun- 

 tain chains, the more general arrangement is represented Plate III. 

 fig. 1. Granite, or the foundation rock, a; gneiss, h; mica slate, 

 c; common slate (called clay slate) ; d d. The transition series, 

 e e. The lower strata with coal, f f. A bed of limestone, or any 

 other rock, in a slate mountain, is represented, x x: in this position 

 it is said to be imbedded ; and if a number of these beds occur, at 

 different intervals, they are said to be subordinate. A bed of con- 

 glomerate, composed of bowlders and fragments of the lower rocks, 

 as at G, is frequently interposed, between slate rocks and transition 

 limestone. 



The unconformable position of unstratified rocks is represented 

 Plate III. fig. 2., where a mass of porphyry a, ranging from c to c, 

 covers the rocks 1, 2, 3, without any conformity to the inclination or 

 form of the lower beds. The lower beds are, however, cut through 

 by veins of porphyry, which indicate that the porphyry had been 

 erupted, in a melted state, through these veins, and poured over the 

 surface of the lower rocks. A similar arrangement of porphyry, 

 which occurs in Norway, will be described in Chap. IX. 



Basalt, either massive or columnar, frequently, covers rocks in an 

 unconformable position. See Plate III. fig. 2. b, d, and h. 



The superincumbent rocks, in this situation, are evidently, of 

 more recent origin than those which they cover : the lower must 

 have been hard and unyielding, when the upper were thrown upon' 



* We have reason to believe that many rocks whicli present no indications of 

 stratification, were, originally, arranged in regular strata. In some limestone 

 rocks, where the stratification is extremely well defined by distinct partings, there 

 occur spaces in which different strata are blended into one mass. These masses 

 are called by the quarrymen, knobs, and are more hard and difficultto work than 

 the stratified limestone, but are equally good in quality. 



