The process employed in cementing the materials of the vitrified forts was then perfectly unique. The processes employed to obtain the necessary heat varied according to circumstances and according to the nature of the materials used. At Sainte-Suzanne and at La Courbe marine salt was used as a flux. Captain Prévot36 thinks that the walls were smeared with a coating of clay, and that as in the baking of bricks spaces were left between so as to produce more intense heat. M. de Montaiglon is of opinion that the buildings were in the first instance erected without the use of any calcareous or argillaceous material, and that glass in a state of fusion was poured over them afterwards, this glass consolidating them and forming with them one indestructible mass. M. Thuot seems much disposed to share this last opinion, but he thinks that some chemical materials such as soda or potash were also used. Yet one other possible solution may be mentioned, a solution which is becoming more and more generally accepted, namely that the granite was not after all really melted, but that the vitrification should either be attributed to the fusion of the argillaceous mass, which has been subjected to an igneous transformation, such as that which often takes place in furnaces for baking bricks and in lime-kilns.37 Whatever explanation we may accept, however, the processes employed certainly bear witness to a much more advanced state of civilization than was acquired in the earliest ages of humanity. We have been led by the great interest and mystery of the subject to dwell longer on it than we intended, and we must hasten to return to prehistoric times with a determination not to transgress again. Fortifications are a proof of combined action leading to a common end; they imply social organization, chiefs to command, workmen to obey. A recent discovery enables us to form a very accurate picture of prehistoric men gathered together not only for purposes of defence, but in a society already rich, industrious, and, if we may so speak, learning to cultivate the arts of peace.