I. I ntroduction . C yrenaica , a province of Eastern Tripoli, is the projection on the northern coast of Africa between the Gulf of Sidra (the Greater Syrtis) on the west and the Bay of Bomba on the east. The name is sometimes used to include Marmarica and the Libyan coast, as far as the still undetermined western frontier of Egypt; but the restriction of Cyrenaica to the wide foreland occupied by the famous Greek colony whose capital was Cyrene renders it a more natural geographical division. It is thus equivalent to the Gebel el Akdar—the Green Hills—of Arab nomenclature; while the Gebel el Akabah—the Abrupt Hills,—its eastward continuation between the Bay of Bomba and the Bay of Salum, is the plateau of Marmarica. Cyrenaica, as thus restricted, is a land of great classical interest: it includes the ancient Pentapolis; the city of Cyrene stood on the northern edge of its plateau; and the Garden of the Hesperides and the River Lethe lay in hollows in the limestone on its coastal plain. Cyrenaica is, however, geologically the least-known area on the shores of the Mediterranean. The geology of the parts of Egypt near the eastern frontier of Cyrenaica is known from the work of Ziittel and Fuchs at Siwa, and from collections made on the coast of Marmarica at Mersa Tobruk by Schweinfurth, and on the Libyan coast by Ball and by Pachundaki at Mersa Matruh, about half-way between Alexandria and the Bay of Salum. The correlation of the rocks on the eastern