so short the period of history by which, from the
experience of man, we have to judge, that we must be persuaded we see
but little of those operations which make any sensible change upon the
earth; and we should be cautious not to form a history of nature from
our narrow views of things; views which comprehend so little of the
effects of time, that they may be considered as nothing in the scale by
which we are to calculate what has passed in the works of nature.

To form an idea of the quantity of the solid land which has been carried
away from the surface of the earth, we must consider our land, with the
view of a mineralist, as having all the soil and travelled materials
removed, so as we might see the terminations of all the strata, where
these are broken off and left abrupt. Now, the generality of those
strata are declined from the horizontal plane in which they had been
formed, and shew that the upper extremity had been broken off and
carried away; and the quantity of that which has been carried away,
since the time of the formation of those strata, so far as may be judged
from the nature and situation of what remains, must be concluded as very
great. This is best to be observed in mountainous countries, where not
only the causes of this destruction of the land are more powerful, but
the opportunities of investigating the effects more frequent, from the
washing away of the loose soil or covering.

The correspondent angles of the valleys among mountains is a subject
of this nature, in which may be perceived a visible waste of the solid
mountain which has those correspondent angles. I am happy to have an
authority so much better than my own observations to give on this
occasion, where the question relates to what is common or general in
these appearances. It is that of M. de Luc, Lettres Physique et Morales,
tom. 2. p. 221. "Mais avant de finir sur les montagnes _primordiales_,
il faut que je revienne a ces _angles saillans et rentrans
alternativement opposes_, qui lorsque Mr. Bourguet les annonca, firent
un si grand bruit parmi les naturalistes qu'on ne douta plus que toutes
les montagnes ne fussent l'ouvrage de la _mer_. Voici ce que c'est que
ce phenomene pretendu demonstratif.

"Lorsqu'on voyage dans les vallees, on va ordinairement en tournoyant;
et quand un angle saillant oblige a courber la route, on trouve assez
souvent un angle rentrant qui lui fait face, et la vallee conserve a
peu pres la meme largeur.