188 PORTLAND STONE QUARRIES. 



island for building. The bur rests upon the dirt bed, which at one 

 time supported a forest ; silicified conifers and cycadea, locally 

 known as bird's nests, occur in some profusion, the trunks of the 

 conifers penetrating through the soft bur above. The roots occur 

 occasionally in the underlying cap, which, with the scull cap 

 terminates the Purbeck series. The dirt bed is only about a foot 

 thick, so enormous has been the pressure to which it was subject. 

 Stumps of trees are found standing five or six feet above, which 

 measure from four to five feet in diameter. They usually bend 

 towards the south-east. In addition to these fossils the dirt bed 

 contains rounded blue stone, which, when broken, gives off a 

 disagreeable smell. The cap, which is from six to eight feet in 

 thickness, intervenes between this and a second dirt bed in which 

 cycadea (bird's-nests) occur. It is a very hard stone, and forms a 

 bed which has to be blasted before removal. The last bed of the 

 Purbeck series is the scull cap, from two to three feet thick, and 

 which rests on the Portland bed. Various means are employed for 

 blasting this hard obstinate cap, the one mostly in vogue at 

 present being dynamite fired by electricity. Several holes can be 

 exploded simultaneously. These are made with a drill of steel from 

 four to five feet in length and about an inch and a-half in diameter 

 with a flattened cutting edge which, being wider than the bar, 

 makes the hole large enough for the bar to clear it. When the drill 

 has been driven down to its full length a bar of iron from eight to ten 

 feet in length with similar cutting edges is substituted. This bar, 

 termed by workmen a jumper, is held by two or three men, and is 

 continuously lifted up and let do\vn with force until the hole is 

 of sufficient depth. The holes are usually placed in a line six or 

 seven feet apart and from eight to twelve feet from the outside. 

 When charged with dynamite and the fuse ready two very fine 

 wires insulated with gutta percha are fixed to a rod of wood which 

 keeps them in position. The detonator, which is placed in a small 

 cartridge of dynamite, is attached to the fuse and secured in such a 

 way as not to allow the admission of water, which is poured into 

 the holes after the introduction of the detonator. Two stout 



