EXTERNAL STRUCTURE OF ROCKS. 



39 



Radiated, when the fibres are broader and flattish and diverging. 

 Lamellar or Foliated, composed of minute plates laid over each 

 other. 



Porous, penetrated by pores. 



Cellular, or Vesicular, when the pores swell into rounded cavities, 



like bladders, as in some lavas. 

 Slaty, or Laminar, composed of straight, parallel, thin plates, or 



laminae. 



The structure of compound rocks, may also be Slaty. 



The external structure of rocks en masse, or considered as moun- 

 tain masses, is as distinct from their internal mineral structure, as the 

 shape of a building from that of the bricks or stones of which it is 

 composed ; though this distinction has been generally overlooked. 

 The external structure of rocks, as forming mountain masses, may be 



Stratified, or stratiform. 



Tabular, or in large plates. 



Columnar, 



Globular, or in spherical masses. 



Massive, or Indeterminate, which includes all unstratified rocks 

 that have no determinate shape. 



Stratified mountains or rocks are those which are composed of 

 layers of stone, laid over each other, and divided by parallel seams 

 like the leaves of a closed book. In these seams or partings, which 

 divide the strata, there are frequently, thin laminae of soft earthy mat- 

 ter; but, sometimes, the surfaces of the upper and lower stratum are 

 so closely joined, that it requires a considerable force to separate 

 them. These layers are denominated strata : they extend through 

 the whole mountain or mass, their length and breadth, being much 

 greater than their thickness. If the thickness of any stratum exceed 

 two or three yards, it is more usually denominated a bed; and if it 

 lie between two beds of stone of a different kind, it is said to be 

 imbedded. Strata, almost always, decline, or dip down to some 

 point of the horizon, and of course rise towards the opposite point. 

 A line drawn through these points is called the line of their dip : 

 another line drawn at right angles to this, marks the course along 

 which the strata stretch out to the greatest extent : — it is called the 

 line of bearing. If a book be raised in an inclined position, with 

 the back resting lengthwise upon the table, the leaves may be sup- 

 posed to represent difl?erent strata ; then, a line descending from the 

 upper edges to the table will be the line of dip, and their direction 

 lengthwise will be the line of bearing ; and the angle they make with 

 the table will be the angle of inclination. Strata, are, however, 

 sometimes curved or bent in both directions, and are frequently bro- 

 ken ; which makes it difficult to ascertain their true position. 



Stratified rocks of sandstone, and beds of clay and marl, are gen- 

 erally admitted to have been deposited by the turbid waters of the 

 sea, or of large rivers or lakes. These sedimentary depositions are 



