592 PROF. 3. W. GREGORY ON [Nov. I9H, 



it is situated only a mile from the border of the plain, there about 

 6 miles wide. Much of the ground has been cultivated, especially 

 in depressions where soft loam has been collected as rain-wash. 

 Near the shrine or marabut of Ahmeda, the loam is covered with 

 slabs of secondary limestone showing that the alluvium is thinner 

 there ; and blocks of limestone resembling the Derna Limestone 

 occur on a ridge, which projects from the downs into the western 

 side of the plain. The surface of this ridge is littered with slabs of 

 efflorescent limestone. 



I could find no exposure of the chert-bearing limestones there ; 

 but, after crossing the nummulitic limestones for a couple of miles, 

 we found that chert-nodules became abundant and then cherty 

 limestones appeared on the surface. The first specimens of these 

 cherts were much altered by secondary action, and the cherts were 

 chalcedonic. At Smuta, there is an outcrop of a shelly siliceous 

 limestone, near the ruins of a Iloman fort. The fossils found are, 

 however, unfortunately indeterminable. 



After crossing the watershed between the plain of Silene and the 

 direct drainage north-westwards to the Mediterranean, we observed 

 that the country became more irregular, owing to the numerous, 

 deep, stream-cut ravines. The rock at the edge of the plateau 

 overlooking the coastal plain contained many casts of shells, but no 

 determinable fossil was collected. Beneath the shelly limestone 

 occurred a limestone breccia, including pebbles of black limestone. 

 A steep gully leads from the plateau to the coastal plain, near the 

 ruin of an old fort, at the height of about 450 feet above the sea, 

 and some 16 miles east from Benghazi. The face of the plateau 

 has been much denuded, but presents the aspect of a fault-scarp. 

 The rocks at its foot are pinkish and white compact limestones, con- 

 taining fragments of Scutella and a thick massive Chjpeaster. As 

 the camels had already gone some distance ahead, I was unable to 

 collect any complete specimens, though the sketches of some of the 

 fossils indicate that the beds are Miocene, and probably Middle 

 Miocene. The Scutella Limestone is exposed farther west in 

 bosses, polished by wind erosion. Nearer Benghazi, these Miocene 

 limestones are covered by a younger limestone, of which the 

 characteristic shell has been determined by Mr. Newton as Ceras- 

 toderma edule (Linn.), and the rock is either late Pliocene or more 

 probably early Pleistocene in age. The older limestone was still 

 visible occasionally in depressions in the coastal plain, and in the 

 solution-cauldrons occupied by the Garden of the Hesperides and 

 the cave known as the River of Lethe. Nummulites have been 

 recorded from this limestone by the Archduke Ludwig Salvator, 1 

 and G. B. Stacey called the rock a ' Tertiary limestone.' 2 The 

 specimens collected there do not contain nummulites, but a virleti- 

 form Ostrea, which suggests the Helvetian age of the rock. 



i ' Yacht-Eeise in den Syrten 1873 ' Prag, 1874, p. 52. 



2 ' On the Geology of Benghazi, Barbary ' Q. J. G. S. vol. xxiii (1867) p. 384 ; 

 he collected some fossils in the limestone which were not specifically determined, 

 and found Cardium edule on the surface (ibid. p. 386). 



