North Africa is a great stronghold of the megalithic civilization, indeed it is thought by some that it is the area in which megalithic building originated. Morocco, Tunis, Algeria, and Tripoli all abound in dolmens and other monuments. Even in the Nile Valley they occur, for what looks like a dolmen surrounded by a circle was discovered by de Morgan in the desert near Edfu, and Wilson and Felkin describe a number of simple dolmens which exist near Lado in the Sudan. Tripoli remains as yet comparatively unexplored. The traveller Barth speaks of stone circles near Mourzouk and near the town of Tripoli. The great trilithons (_senams_) with holes pierced in their uprights and 'altar tables' at their base, which Barth, followed by Cooper in his _Hill of the Graces_, described as megalithic monuments, have been shown to be nothing more than olive-presses, the 'altar tables' being the slabs over which the oil ran off as it descended. True dolmens do, however, occur in Tripoli, and Cooper figures a fine monument at Messa in the Cyrenaica, which appears to consist of a single straight line of tall uprights with a continuous entablature of blocks similar to that of the outer circle at Stonehenge. Algeria has been far more completely explored, and possesses a remarkable number of megalithic monuments. Many of the finest are situated near the town of Constantine. Thus at Bou Nouara there is a hill about a mile in length which is a regular necropolis of dolmen-tombs. Each grave consists of a dolmen within a circle of stones. The blocks are all natural and completely unworked. The circle consists of a wall of stone blocks so built as to neutralize the slope of the hill and to form a level platform for the dolmen. Thus on the lower side there are three courses of carefully laid stones rising to about five feet, while on the upper side there is only one course. The diameter of the circles varies from 22 to 33 feet. In the centre of the circle lies the dolmen with its single long cover-slab. This usually rests on two entire side-slabs, the ends being filled up either with entire slabs or with masonry of small stones. In rare cases the side-slabs are replaced by masonry walls. The average size of the cover-slab is 6-1/2 by 5 feet. The dolmen itself is, of course, built directly on to the platform, and the space between it and the circle is filled up with rough stones. The orientation of the dolmens varied considerably, but the cover-slab was never placed in such a way that its length ran up the hill-slope, probably because in moving the slab into place this would have been an awkward position.