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 of the incorporated blocks of slate. Some people have explained these patches of black mica as segregation patches; that is, areas in which certain minerals have segregated by mutual attraction when the rock was crystallizing.

Dykes. — Smaller masses of liquid rock manage to squeeze into the earlier-formed rocks as arms or dykes. Magnificent examples of such dykes occur in the slate exposed on the foreshore at Sea Point, and were reported on by Clarke Abel in 1818; Hutton used these examples in his famous struggle for the admission that rocks could be intruded.

Dykes may become extended arid squeeze up through fissures in the rocks and along bedding planes, widening their channel by absorbing the walls, and so a tongue of liquid rock may travel far from the parent source. A dyke that comes to follow a horizontal plane is called a sill or sheet.

A dyke may in its course come so near the surface that the impelling force behind it is able to lift up the cover of rocks, and hence a large dome-shaped chamber becomes formed, into which the liquid rock pours and solidifies. At other places the solubility or fusibility of the rocks is more at a particular place than at others, and here the same bulging of the dykes occurs, owing to the liquid magma eating out a great chamber for itself. These expanded dykes are called laccolites.

A laccolite may be a single solid expansion of lava, as in the dolerite laccolites of the Insiswa at Mount Ayliff, Mount Currie, and several others on the Drakensberg plateau; smaller ones are to be seen at Cradock and Burghersdorp. Another form is where the central expansion is solid, but round the edge is a fringe of