As early as 1908, Breuil(34) discovered in the interior of the cave back of the grotto some quartzites worked into Acheulean types, proving that the cavern was entered in Acheulean times. Obermaier,(35) in the course of three years' work, has found that the floor of the grotto was possibly used as a flint-making station in Acheulean and, possibly, in Chellean times. The culture section which he has revealed here under the direction of the _Institut de Paléontologie humaine_ can be compared only with that which Commont has found on the 'terraces' of the Somme at St. Acheul. The difference is that in the shelter of the Castillo grotto the climate is recorded only through the changing forms of animal life which are mingled around the fire-hearths and with the flints in the ascending levels. (13) Eneolithic Age. Small, triangular dagger in copper. (12) Azilian. Flint industry--Age of the Stag. (11) Upper Magdalenian. Artistic engravings on stag-horn. (10) Lower Magdalenian. Flints and fine engravings on bone. Reindeer bâton. (9) Archaic Solutrean. _Feuilles de laurier_, retouched on one side only. (8, 7, 6) Upper Aurignacian in three layers. Remains of the reindeer and _burins_. (5) Lower Aurignacian. Implements of stone and bone. Remains of an infant. (4) Upper Mousterian. Rich in small implements and large tools of quartzite. Merck's rhinoceros very abundant. (3) Typical Mousterian flints and quartzites. Merck's rhinoceros. (2) Early Mousterian industry. Bones of cave-bear and Merck's rhinoceros. (1) Acheulean flints. [Illustration: FIG. 79. Stratigraphic section showing the archæologic layers of the great grotto of Castillo. After Obermaier.] The entrance to this grotto is on the side of a high hill overlooking the valley and might easily have been barricaded against attack. In early Acheulean times, when the flint workers were on the very floor of the grotto, the lower entrance of the cavern was still open, leading far into the heart of the mountain. The successive accumulations of débris, cave loam, fire-stones, bones, and innumerable flints, together with great blocks falling over the entrance of the cavern, reached a height of 45 feet, so that during the Upper Palæolithic only the upper entrance to the cavern was used by the artists of Magdalenian times. The subsequent Azilian and Eneolithic cultures were crowded under the very roof of the grotto at the sides. This station, repaired to and then abandoned by tribe after tribe over a period estimated at present as not less than 50,000 years, is a monumental volume of prehistory, read and interpreted by the archæologist almost as clearly as if the whole record were in writing. The first positive evidences of the use of fire are the layers of charred wood and bones frequently found in the industrial deposits of early Acheulean times. GEOGRAPHIC AND CLIMATIC CHANGES