Bearing this in mind, we have no difficulty in beginning our history of megalithic remains with the rude stone cists, generally called kistvaens, which are found in sepulchral tumuli. Sometimes these consist of only four, but generally of six or more stones set edgeways, and covered by a cap-stone, so as to protect the body from being crushed. By degrees this kistvaen became magnified into a chamber, the side stones increasing from 1 or 2 feet in height to 4 or 5 feet, and the cap-stone becoming a really megalithic feature 6 or 10 feet long, by 4 or 5 feet wide, and also of considerable thickness. Many of these contained more than one funeral deposit, and they consequently could not have been covered up by the tumuli till the last deposit was placed in them. This seems to have been felt as an inconvenience, as it led to the third step, namely, of a passage communicating with the outer air, and formed like the chambers of upright stones, and roofed by flat ones extending across from side to side. The most perfect example of this class is perhaps that in the tumulus of Gavr Innis in the Morbihan. Here is a gallery 42 feet long and from 4 to 5 feet wide, leading to a chamber 8 feet square, the whole being covered with sculptures of the most elaborate character. A fourth stage is well illustrated by the chambers of New Grange, in Ireland, where a similar passage leads to a compound or cruciform chamber rudely roofed by converging stones. Another beautiful example of the same class is that of Maeshow in the Orkneys, which, owing to the peculiarity of the stone with which it is built, comes more nearly to the character of microlithic art than any other example. It is probably among the last if not the very latest of the class erected in these isles, and by a curious concatenation of circumstances brings the megalithic form of art very nearly up to the stage where we left its microlithic sister at Mycenæ some two thousand years before its time. All this will be made clearer in the sequel, but meanwhile there are one or two points which must be cleared up before we can go further. Many antiquaries insist that all the dolmens[58] or cromlechs,[59] which we now see standing free, were once covered up and buried in tumuli.[60] That all the earlier ones were so, is more than probable, and it may since have been originally intended also to cover up many of those which now stand free; but it seems impossible to believe that the bulk of those we now see were ever hidden by any earthen covering.