An ancient dolmen used as a cellar

I´m really curious about this image I´ve found in the book FACTS ABOUT CHAMPAGNE AND OTHER SPARKLING WINES, COLLECTED DURING NUMEROUS VISITS TO THE CHAMPAGNE AND OTHER VITICULTURAL DISTRICTS OF FRANCE, AND THE PRINCIPAL REMAINING WINE-PRODUCING COUNTRIES OF EUROPE. BY HENRY VIZETELLY. London, 1879.

It´s an ancient dolmen but you can see a door at the ¨entrance¨ of much modern manufacture. This is an excellent example of  ¨appropriation¨, and change of use. Below, all I could find as a reference: 

 Chevalier of the Order of Franz Josef. Wine Juror for Great Britain at the Vienna and Paris Exhibitions of 1873 and 1878. Author of “The Wines of the World Characterized and Classed,” &c. While visiting the vineyards of Varrains and Chacé we came upon a couple of dolmens—vestiges of the ancient Celtic population of the valley of the Loire singularly abundant hereabouts. Brézé, the marquisate of which formerly belonged to Louis XVI.’s famous grand master of the ceremonies—immortalized by the rebuff he received from Mirabeau—boasts a noble château on the site of an ancient fortress, in connection with which there are contemporary excavations in the neighbouring limestone, designed for a garrison of 500 or 600 men. Beyond the vineyards of Saint-Florent, westward of Saumur and on the banks 147 of the Thouet, is an extensive plateau partially overgrown with vines, where may be traced the remains of a Roman camp. Moreover, in the southern environs of Saumur, in the midst of vineyards producing exclusively white wines, is one of the most remarkable dolmens known. This imposing structure, perfect in all respects save that one of the four enormous stones which roof it in has been split in two, and requires to be supported, is no less than 65 feet in length, 23 feet in width, and 10 feet high.

http://www.gutenberg.org/files/20889/20889-h/20889-h.htm