They vary greatly in coarseness . Often submarine , they may contain shale , sandstone , grit and other sedimentary material , and are occasionally fossiliferous . They are black , dark green or red in color ; vary greatly in coarseness , some being full of round spongy bombs a foot or more in diameter , and , being often submarine , may contain shale , sandstone , grit and other sedimentary material , and are occasionally fossiliferous .