In the north of the Fayûm the series is characterized by the presence of one or more very constant well-marked beds of hard calcareous sandstone, which almost invariably weather into huge globular masses. These masses should be regarded as huge weathered-out concretions, rather than as water-rounded blocks, although no doubt in many cases their roundness has been increased by the action of the waters of Lake Moeris as the level of the latter gradually fell, and possibly still earlier during the invasion of the Pliocene sea; from the latter time also may date the millions of parallel vertical borings with which these and other exposed rocks are often perforated. In the various places where one of these beds forms the present surface of the desert the concretions may be seen in different stages of exposure, from the initial, where only just the tops are laid bare, to the final stage where the globes are left completely weathered out, as seen in the illustration (Plate VII). The appearance of the desert when covered for many square kilometres with thousands of these blocks is more easily imagined than described. The lower beds of the Birket el Qurûn series form the island Geziret el Qorn, and consist of clays and sandstones containing a considerable number of organic remains. These beds were collected from and examined by Schweinfurth[44] in 1879, the mollusca being subsequently described by Mayer-Eymar,[45] while the vertebrate remains, which included cetacean bones and numerous fish-teeth, were submitted to Dames. The following species were determined by Mayer-Eymar, who indicated that the fauna as a whole had a Bartonian aspect[46]:— UPPER BED. _Ostrea plicata_, Defr. _Arca Edwardsi_, Desh. _Lucina pomum_, Duj.[47] _Lucina_ cfr. _tabulata_, Desh. _Cardium Schweinfurthi_, May.-Eym. _Cytherea Newboldi_, May.-Eym. _Tellina pellucida_, Desh. _Mactra compressa_, Desh. _Corbula pyxidicula_, Desh. _Calyptræa trochiformis_, Lam. _Turritella angulata_, Sow. _Ficula tricarinata_, Lam. LOWER BED. _Astrohelia similis_, May.-Eym. _Goniastræa cocchii_, d’Achiardi. _Heliastræa acervularia_, May.-Eym. _Heliastræa Ellisi_, Defr. (_Astræa_). _Heliastræa flattersi_, May.-Eym. _Ostrea digitalina_, Dubois. _Ostrea gigantea_, Sol. _Ostrea longirostris_, Lam. _Ostrea producta_, Delb. et Raul. _Isocardia cyprinoides_, Braun. _Turritella carinifera_, Desh. _Turritella transitoria_, May.-Eym. _Turritella turris_, Bast. _Turbo Parkinsoni_, Defr. _Pleurotoma_, sp.