SOILS AND THEIR CHARACTERISTICS 7 



preserve great compactness, as well as the conglomerates or pudding- 

 stones, which consist of pebbles held together by some natural 

 cement, are difficult to remove from the bottom of water. Their 

 removal, however, may be successfully obtained by hammering 

 or even by blasting or otherwise, by powerful dredges in which 

 the buckets are provided with strong steel teeth. It is in the disin- 

 tegrated and soft rocks that the method of breaking the material 

 by hammering is considered the most economical. In many instances, 

 disintegrated rock has been successfully broken by hammering. 

 Of the different types of dredges, the one that gives, the best result 

 in digging through disintegrated rock and conglomerates is the 

 dipper dredge; although those of the clamshell and ladder types 

 have also been employed with advantage. The bucket of the dipper 

 dredge used in these soils is armored with four strong steel teeth. 

 These are forced into the conglomerate or disintegrated rock and by 

 pulling up the bucket the material is dislodged and the debris fall- 

 ing into the bucket is raised to the surface. But when these compact 

 materials are encountered at such a depth that the dipper dredge 

 cannot work to advantage, both the ladder and clamshell dredges 

 are found more adaptable. When the ladder dredge is used the 

 buckets are provided with strong steel teeth and the endless chains 

 driving the buckets are run with great velocity in order that 

 the teeth may engage the compact material with great force. By 

 means of a ladder dredge with armored buckets even large-sized 

 stones can be removed from the bottom. Thus stones of over 

 1 cu.m. were picked up by the buckets of a ladder dredge in the 

 harbor of Dunkerque in the English Channel. When the clam- 

 shell dredge is used the bucket is either entirely formed by strong 

 tines or the edges of the bucket are provided with tines. The grab 

 bucket lowered to the bottom at great speed will strike the rock 

 with force and the tines will penetrate into the mass. By closing 

 the bucket the rock will be broken and the debris will thus be raised 

 to the surface. 



Gravel. Gravel is a soil chiefly composed of small hard water- 

 worn smooth fragments of rock ordinarily mixed with sand. When 

 gravel is encountered in thick layers at the bottom of rivers or 

 bays, it is generally a very compact material, so as to be considered 

 almost as hard as any conglomerate. However, when it is attacked 

 by the strong teeth of the buckets of dredges of various types, and 

 a furrow has been dug through the bank, the small round pebbles 



