The addition or diminution of a mountain would not be sufficient to produce a diversity in a planet: though the change of a very few inches would be able to destroy the identity of some bodies.

It will be impossible to account for this, but by reflecting that objects operate upon the mind, and break or interrupt the continuity of its actions not according to their real greatness, but according to their proportion to each other: And therefore, since this interruption makes an object cease to appear the same, it must be the uninterrupted progress o the thought, which constitutes the imperfect identity.
This may be confirmed by another phenomenon.

A change in any considerable part of a body destroys its identity; but it is remarkable, that where the change is produced gradually and insensibly we are less apt to ascribe to it the same effect.

The reason can plainly be no other, than that the mind, in following the successive changes of the body, feels an easy passage from the surveying its condition in one moment to the viewing of it in another, and at no particular time perceives any interruption in its actions.

From which continued perception, it ascribes a continued existence and identity to the object.
But whatever precaution we may use in introducing the changes gradually, and making them proportionable to the whole, it is certain, that where the changes are at last observed to become considerable, we make a scruple of ascribing identity to such different objects.