Silbury Hill is itself an artificial conical mound, the largest in England, 170 feet high, on which were originally no less than 650 upright stones, of which only twenty are still standing, surrounded by a trench. In the centre of the circle of stones a single menhir of great height still remains with three others sloped so as to form a kind of crypt. The megalithic monuments of Stonehenge, which are probably better known than any others in the world, are perhaps also the most curious. The group is supposed to have originally consisted of an outer stone concentric circle some one hundred feet in diameter, formed by thirty piers of solid masonry, of which about twenty can still be made out, some few standing, others lying broken upon the ground. This outer circle enclosed a second of similar shape but lesser diameter, within which again were taro elliptic circles, the outer consisting of ten or twelve sandstone blocks some twenty-two feet high, standing in pairs, each pair united by a slab laid horizontally across, so as to form a trilithon. The inner ellipse was formed by nineteen upright masses of granite, within which was the famous slab of blue marble, by many supposed to have been an altar. The pillars and lintels of the outer portico, and those of the trilithons, are fitted together with the greatest skill, with tenons and mortices, a remarkable exception to the general rule with megalithic monuments. Everywhere in the neighborhood of Stonehenge, as far as the eye can reach, are tumuli, all nearly equidistant from the principal group of monuments, a fact which has led many archæologists, including Henry Martin, to look upon. Stonehenge as a temple surrounded by a necropolis. Excavations at Stonehenge have yielded a few human bones which have escaped the flames, with some stone and bronze weapons.