The piercing of a road tunnel in the flank of a limestone (Malm) anticline in the Neuchâtel Jura uncovered karstic forms transformed for the most part, by decarbonated soils. Mineralogical analysis of these latter, through the use of X-ray diffraction, reveals a great analogy with the surface soils. At more than 200 meters depth, the same allochtone mineralogical suite of aeolian origin which constitutes the largest part of the soils of the High Jura Mountains in Switzerland, is found: an abundance of ferriferous chlorite, and of quartz, plagioclase and potassic feldspar. The various factors favouring this deep infiltration are discussed.