ALUM SHALE. 129 



Having taken this general view of the alum shale, and the beds 

 subordinate to it, as they appear at Boulby, let us notice more parti- 

 cularly the nature of the different beds, in their order. 



The first and principal member of this series is the main bed of 

 ALUM SHALE, marked (k) in the Section. This bed varies from about 

 180 to 200 feet in thickness. It has the laminated or slaty structure, 

 parting horizontally into plates, which are more or less thin, accord- 

 ing as they are higher or lower in the bed, the lowest parts of it being 

 the least fissile. This difference, however, may result from a difference 

 of exposure and of pressure, more than a real diversity of structure ; 

 for the most compact blocks, taken from the lower part of the bed, 

 soon split into thin laminae, when exposed to the atmosphere, and 

 released from the pressure of the superincumbent mass. The slaty 

 fragments, under the continued action of the atmosphere, will split 

 again into very thin lamellce, which are blown about with the winds. — 

 Besides these horizontal partings, this rock is divided by vertical 

 seams. These are disposed in two sets, the seams of each set being 

 parallel to one another, but crossing those of the other set obliquely ; 

 and hence, by the intersections of these two sets of seams, the rock 

 is split into rhomboidal blocks. The seams, however, are not placed 

 at regular distances, so that the solid rhomboids are more or less 

 oblong; and since these vertical fissures, like the horizontal partings, 

 diminish in number as we descend, the rhomboidal sections grow 

 larger and more compact, according to their depth. Sometimes the 

 two sets of vertical seams cross each other nearly at right angles, 

 dividing the rock into parallelopipeds, rather than solid rhomboids. 

 We may add, that, of the intersecting veins or seams, some are princi- 

 pal veins which may be traced to a great distance; others are subordi- 

 nate, running only a short way in the rock. 



When broken in the direction of the laminae, the alum schist pre- 

 sents a smooth fracture, and very fine grain, and feels rather soft and 



2 li 



