The more we think of the difficulties vitrification presents, the greater is our astonishment. How was the fusion achieved of elements so refractory alike in their structure and in the resistance offered by accumulated masses of material? By what processes was heat brought up to the 1300 degrees necessary for the fusion of granite? The incineration and fusion of the materials of which the vitrified forts are made, especially the granite ones of La Creuse and the Côtes du Nord, bear witness, says Daubrée, to a surprising skill and knowledge of the management of fire in those who burned them, but these qualities were manifested also in extremely ancient metallurgical operations. It is quite impossible to suppose the vitrification to have been the result of a conflagration. No fire, whether accidental or the work of an incendiary, could be powerful enough to produce such results. The use of petroleum in the most terrible conflagrations of our own time—those of the Commune in 1871, for instance—did calcine and disintegrate stone, but I know of no case of vitrification. The Keramic Museum of Sèvres contains several specimens which present very notable differences to each other. Those from Château-Gontier are formed of very close-grained quartzite granite of a greenish color streaked with black. The conglomerate welding there together is a vitrified scoria full of very small bubbles made by the escape of gas which had not had sufficient strength to get out. The block from Sainte-Suzanne (Mayenne) consists of quartz mixed with half calcined grains of feldspar, bleached by the action of fused glass, which once introduced filled up as it congealed all the vacant spaces with a vitreous substance of light greenish-white color. The fractures are green and bright, and are dotted with white points, which are all that is left of the stones after their disintegration in the grip of a heat that was alike intense and rapid in its action. The fragments brought from Scotland differ from those just described. They consist of small pieces of granite completely merged in a thick paste with which they form the mass, the whole breaking together when it does break; and the melted matter seldom has any bubbles in it.35