We are thus led to suppose, that some parts of our earth may have undergone the vicissitudes of sea and land more than once, having been changed from the summit of a continent to the bottom of the sea, and again erected, with the rest of that bottom, into the place of land. In that case, appearances might be found to induce natural philosophers to conclude that there were in our land primary parts, which had not the marine origin which is generally to be acknowledged in the structure of this earth; and, by finding other masses, of marine origin, superincumbent upon those primary mountains, they might make strange suppositions in order to explain those natural appearances. Let us now see what has been advanced by those philosophers who, though they term these parts of the earth _primordial_, and not _primitive_, at the same time appear to deny to those parts an origin analogous to that of their secondary mountains, or strata that are aquiform in their construction. M. de Luc, after having long believed that the strata of the Alps had been formed like those of the low countries, at the bottom of the sea, gives an account of the occasion by which he was first confirmed in the opposite opinion.[26] Like a true philosopher, he gives us the reason of this change. [Note 26: Lettres Physique et Morales sur l'Histoire de la Terre, tom. 2. pag. 206.] "Ce fut une espèce de _montagne_ très commune, et que j'avois souvent examinée qui dessilla mes yeux. La pierre qui la compose est de la classe appellée _schiste_; son caractère générique est d'être _feuilletée_; elle renferme _l'ardoise_ dont on couvre les toits. Ces _feuillets_ minces, qu'on peut prendre pour des _couches_, et qui le font en effet dans quelques pierres de ce genre, rappelloient toujours l'idée vague de dépôts des eaux. Mais il y a des masses dont la composition est plutôt par fibres que par feuillets, et dont le moëllon ressemble aux copeaux de bois d'un chantier. Le plus souvent aussi les feuillets sont situés en toute suite de sens dans une même _montagne_, et quelquefois même verticalement, Enfin il s'en trouve de si tortillés, qu'il est impossible de les regarder comme des dépôts de l'eau.