412 8. V. Wood — Origin of the Loess. 



occupies the slopes, and is termed " limon biefeux." Near Amiens, 

 he says, the limon which covers the valley slopes is generally much 

 thinner than that at the foot of the slopes, or on the plateaux, and 

 the pits on the latter " always encounter beneath several metres of 

 purer limon, the limon biefeux with flint fragments covering either 

 the bief a silex or the chalk. Upon the slopes of the hills, or at 

 their feet, the brickpit sections always show this biefeux bed with 

 eclats of flint in contact with the chalk, or with the gravels under- 

 lying the limon." This " biefeuse " base of the limon, he adds, 

 " ravines " moi-e or less deeply the subjacent deposits. 



Next he observes that the limon biefeux contains along with the 

 flint fragments some entire flints derived from the formation on 

 which it reposes, which if extracted with care are found to be split 

 in all directions, but have their fragments held together by a feeble 

 cement of the limon ; the surfaces of these fragments thus held being 

 whitened, and rendered porous and opaque. This, he says, occurs 

 equally whether the flints are obtained from the base of the limon, 

 where this occupies the surface, or from beneath a thickness of several 

 metres of the limon, where they are protected by it from all present 

 influence of atmospheric conditions ; and having submitted to an 

 artificial refrigeration, equal to the greatest cold of our present 

 winters, flints steeped for some time in water without producing by 

 it any change in their condition, he infers that recourse must be had 

 to the rigour of polar winters and to " a glacial pressure " to explain 

 their splintering. 



M. de Mercey then describes the materials into which specimens 

 of this limon when dissolved in water, thoroughly shaken, and 

 allowed to precipitate, became resolved when examined under the 

 microscope. This examination showed the limon to be nearly all 

 made up of " grains of quartzose sand reduced to an impalpable 

 tenuity." The first precipitation from the solution (and which was 

 almost immediate), represented about voths of the limon assayed; 

 and was composed of transparent grains of quartz of the average 

 dimension of -aVth of a millimetre, with which were associated some 

 other grains appearing to belong to different substances, and notably 

 to oxide and sulphate of iron. The next precipitation from the 

 solution represented about another -i-Q-ths of the limon assayed ; and 

 was composed of grains of quartz, mostly transparent, averaging about 

 •Jg-th of a millimetre ; while the remaining xoth, which remained a 

 long time in suspension, was composed of grains of quartz averaging 

 T-ffo-th of a millimetre. This showed that the limon was composed 

 almost entirely of silex, a feeble proportion only of other substances, 

 alumina, lime, oxide of iron, etc., being present ; and led M. de 

 Mercey to the conclusion that it originated in an impalpable powder- 

 ing (levigation) of the flints, and resulted from a reconstruction of 

 the "bief a silex," which at the time of the formation of the limon 

 constituted the superficial soil of nearly all the region, except that 

 occupied by the ancient alluvia, or by the chalk exposed in the 

 excavation of the valleys. A washing of this red " bief " showed 

 him that though inappropriately called clay, it, like the limon, is 



