1866. J . FISHER WARP. 555 



for my present purpose, an advantage over calcareous strata, in that 

 they are not soluble by water, and the phenomena therefore are less 

 complicated. As far as my observations extend, I have found that 

 cylindrical pits and pipes are generally confined to soluble beds, and 

 that the normal form of the cavities in clays, sands, and gravels is 

 that of troughs or furrows*. They are usually filled vsdth materials 

 derived from some neighbouring higher ground, and consequently 

 generally differing from the subjacent stratum at that spot ; and it is 

 by the admixture of the contents of the furrows with the material of 

 the subjacent stratum that the warp, which is derived from the two 

 conjointly, comes to contain materials mixed in a different propor- 

 tion from that in the subjacent bed. 



These furrows must be important indications of the mode of de- 

 nudation of those surfaces where they occur ; nevertheless, being 

 simply the tool-mark of the last agent which has moulded the sur- 

 face, if we could determine from them what that was, we need not 

 exclude other and different ones which may have preceded it. 



For the sake of a name I shall call the materials which fill these 

 furrows the " trail." And I will now give a few examples of them 

 in diagram. 



Fig. 1. — Section of a furrow which crosses the Tendring Hundred 

 Railway, near Great Bentley Church, Essex. 



- — — — — — — — . Eailway. 



It is 7 feet deep, full of grey sandy and gravelly clay {h), eroded in brown sands 

 (c). The warp (a) covers it. There is a layer of pebbles at the bottom of 

 the furrow, and roots have penetrated throughout it. The gravel is more 

 angular than that of the glacial drift from which it has been derived, many of 

 the rounded pebbles having been broken up into angular fragments. 



At the gravel-pit. Ballast Quay Farm, near Wivenhoe, Essex, two 

 masses of trail have been left, being too clayey for use. They are 

 parallel to each other ; one of them is fifty yards long, the other not 

 much less. Erosion has gone on beneath them, which is shown by 

 the pebbles of the warp, and also of the trail, lying in festoons. 

 They are inclined at a small angle to the surface-drainage. They 

 occur in sandy glacial gravel. 



* For examples of these troughs, see figs. 1 and 5. London clay dug for 

 ballast at Bentley Tey, near G-reat Bentley, to the depth of 5 feet, exposed a 

 section of a furrow ten yards long and a yard wide, running in the direction of 

 the drainage of the surface. 



