It may therefore be possible to analyse them making use of the hypotheses and the classification which he developed. The engravings of the Djelfa region appeared to him like "foreign works, which are a copying (always of inferior quality) from those of south Oran", (p. 193), a region which for the author was "the principal centre of the rock art of the pre-Saharan regions." Some belong to the earliest stage of the large-scale Hartebeest school, like "The Apollo of Ouled Naïl", others are more recent or indeed (stylistically) more decadent.