PRINCIPLES OF STRATIFICATION. 



45 



fall into is, mistaking an under for an upper stratum. Suppose a hill 

 to be covered with vegetable soil, and, that a quarry or pit being 

 made in it near the bottom, as at «, Plate I. fig. 1., the rock was dis- 

 covered to be sandstone : if another pit were sunk, near the summit 

 at 6, which cut into liemstone, it might be supposed, because the lime- 

 stone is met with at a higher level, that it lies over the sandstone stra- 

 tum, when it is in reality below it. The young observer, who has 

 not a clear notion of this, maybe said not yet to have passed {he pons 

 asinorum of the geologist. 



In calcareous mountains of vast magnitude, as those in the 

 Swiss and Savoy Alps, the enormous beds of limestone are often 

 intersected by regular seams, which cut through the whole bed, in a 

 direction nearly perpendicular to that of the true strata seams, or 

 make very oblique angles whh them. These partings or seams are 

 sometimes nearly vertical, when the strata are almost horizontal. The 

 cliffs and escarpments of these mountains being lofty, and much ex- 

 posed to the action of the atmosphere, the vertical seams enlarge, and 

 are, often, more conspicuous than the strata seams ; hence, without 

 great attention the observer may describe the strata of a mountain as 

 being perpendicular, when, in reality, they are nearly horizontal. To 

 add to the difficulty, it, very frequently, happens, that a calcareous 

 deposition, like a coat of plaster, covers the face of a rock : this has 

 been formed, by moisture running over the surface, and depositing 

 calcareous particles upon it. This deposition, sometimes, conceals 

 the partings or seams of the stratification, as completely as a coat of 

 plaster covers the rows of brick in a building. The vertical seams 

 or partings are also sometimes open, and sometimes form parallel 

 ridges, which efface the appearance of the strata seams in one part 

 of a rock, but not in the other ; and in such instances we have a 

 mountain mass in which the strata are, apparently, partly horizontal 

 and partly vertical. See Plate I. fig. 5. Inattention to this circum- 

 stance, I am convinced, has, sometimes, deceived the eye of M. 

 Saussure, one of the most diligent and accurate of observers. 



The regular partings or cleavages in many slate rocks which inter- 

 sect the beds, nearly at right angles to their dip or inclination (See 

 Plate III. fig. 1. dd), have, often, been mistaken for strata seams, and 

 have led geologists of some eminence to draw very erroneous infer- 

 ences. The thick beds of transition or mountain limestone which 

 compose a great part of Ingleborough, and other adjacent mountains 

 in the district called Cravon, in Yorkshire, generally dip at a mode- 

 rate inclination towards the south-east ; the lower beds rest on coarse 

 slate, which has in reality the same inclination as the limestone, but 

 as the under part of the slate is often concealed, the vertical partings 

 are mistaken for strata seams. This limestone is described by Pro- 

 fessor Playfair as resting on vertical beds of slate ; and he draws 

 several important conclusions respecting the elevation of the beds of 

 slate, and its action on the superincumbent beds of limestone ; where- 



