In this basin have been successively deposited the tertiary strata, in the centre of which Paris is situated. On a circular space bounded by the towns of Laon. Mantes, Blois, Sancerre, Nogent-sur-Seine, and Epernay, these strata appear at the surface, and conceal the chalk, but on the other side of the towns we have mentioned, the edge of the basin being passed, the chalk is found generally on the surface." If we look at the order in which the tertiary strata occur, we shall then comprehend the obstacles M. Mulot had to overcome, and the probability of the ultimate success of his undertaking. Leaving unnoticed the surrounding hills, we will examine the nature of the soil which composes the Plain of Grenelle. On the surface it is formed of gravel, pebbles, and fragments of rock, which have been deposited by the waters at some period anterior to any historical record. Below this surface M. Mulot knew, by geological inductions and previous experience, that at Grenelle marl and clay would be found in the place of the limestone which in general forms the stratum immediately beneath. M. Mulot was aware that he must bore about four hundred and forty yards in depth, before he should meet with the sources which flow in the gravel below the limestone, and supply the wells of St. Ouen, St Denis, and Stains. Beneath the marl and clay, the boring-rods had to perforate pure gravel, plastic clay, and finally chalk, which forms the bottom of the basin in which the tertiary strata have been deposited. No calculations or geological knowledge could determine the thickness of this stratum of chalk, which, from its powers of resistance might present a nearly insurmountable obstacle. The experience obtained in boring the wells of Elbeuf, Rouen, and Tours, was in this respect but a very imperfect guide.