The wide distribution of these simplest stone monuments is exceedingly interesting. They occur in Denmark and Sweden, in North Germany and Holland, in Great Britain and France, Portugal and Spain, in North Africa, in the Ægean Islands, in Palestine and farther eastward, in Thrace and Crimea, along the eastern shore of the Black Sea. They are very numerous in India.[89] Throughout this wide extent they agree not only in general form and structure, but also in certain interesting details. For instance, the oriental and southern dolmens frequently have a round opening in the upper part of the slab closing the entrance, corresponding to the wide opening above the door of the Scandinavian dolmens. The difference in the form of the opening may be explained by the difficulty of cutting a circular opening in the hard granite rocks of the northern area. There was a general unity of thought in essentials, especially in those oldest forms. There was much diversity in execution or expression in later structures. Some of them took the form of pyramids in Egypt. In Mycenæ we find the "Tomb of Atreus," a magnificent building in the form of a beehive. The large chambers, "Giant Chambers" or _Riesenstuben_ of northern Europe, especially of France, are connected with the older small dolmens by many intermediate forms. For example, if another pair of stones is added to the sides of a fair-sized dolmen, we have a chamber six to eight feet in length. Such dolmens always have a covered entrance to the doorway of at least two pairs of upright stones extending out through the tumulus. Then the number of stones in the sides of the chamber is increased to seven, eight, or nine; and the entrance passage is at right angles to the main axis of the chamber, giving a rude T-shaped form to the whole structure. The number of stones in the roof of the chamber increases with its length. Chambers fifteen to twenty feet long are not uncommon, a length of twenty to thirty feet is rare, a very few attain forty feet. The height was between five and seven feet. The inner surface of the great stones forming the sides of the chamber is fairly flat. It could have been no easy matter to find in any region a sufficient number of suitable great blocks of the right form. They evidently had some method of splitting large boulders. In some chambers both halves of the same block have been found. These blocks could have been split by heat or by freezing water in a groove or by wooden wedges. But we do not know the exact method. Near the top the blocks often failed to meet exactly. Large holes were filled with bits of wall of small stones and small chinks were stuffed with clay and moss. It is surprising to find that these smaller and larger chambers were erected without any deep foundation for the upright stones. Many of them have fallen from the heaving of the frost. The monuments were generally adequately protected against this by the thick tumulus.