98 THE OLD RED SANDSTONE. 
remarkably abrupt — an excellent wedge, both in consistency 
and form — instead of having acted on the surrounding dep- 
ositions, as most of the south country traps have done that 
have merely issued from a vent, and overlaid the upper 
strata, it has torn up the entire formation from the very bot- 
tom. Imagine a large wedge forced from below through a 
sheet of thick ice ona river or pond. First the ice rises in 
an angle, that becomes sharper and higher as the wedge 
rises; then it cracks and opens, presenting its upturned edges 
on both sides, and through comes the wedge. And this isa 
very different process, be it observed, from what takes place 
when the ice merely cracks, and the water issues through 
the crack. In the one case there is a rent, and water dif- 
fused over the surface; in the other, there is the projecting 
wedge, flanked by the upturned edges of the ice; and these 
edges, of course, serve as indices to decide regarding the 
ice’s thickness, and the various layers of which it 1s com- 
posed. Now, such are the phenomena exhibited by the 
wedge-like granitic ridge. The Lower Old Red Sandstone, 
tilted up against it on both sides, at an angle of about 
eighty, exhibits in some parts a section of well nigh two 
thousand feet, stretching from the lower conglomerate to 
the soft, unfossiliferous sandstone, which forms in Ross and 
Cromarty the upper beds of the formation. There is a 
mighty advantage to the geologist in this arrangement 
When books are packed up in a deep box or chest, we have 
to raise the upper tier ere we can see the tier below, and this 
second tier ere we can arrive at a third, and so on to the bot- 
tom. But when well arranged on the shelves of a library, 
we have merely to run the eye along their lettered backs, 
and we can thus form an acquaintance with them at a glance, 
which in the other case would have cost us a good deal of 
