A heterogeneous plastic composition suitable for forming rubber-like articles is prepared by forming a first composition containing carbon black and a polymeric material, this first composition being plastic but vulcanizable to the elastic state characteristic of soft rubber, forming a second composition containing a polymeric material and also if desired containing carbon black, this second composition being of substantially greater plasticity, preferably at least 20 points, as measured by the Mooney plastometer, than the first composition, but also vulcanizable to the elastic state characteristic of soft rubber, and incorporating the first composition as discrete dispersed particles throughout the second composition as a matrix. Portions of the first composition may be added to a mass of the second composition while the latter is being masticated and a major proportion of the carbon black of the first composition may be incorporated therein by adding it to a latex of the polymeric material in the first composition, coagulating the latex and subjecting the coagulum to shear treatment prior to the incorporation of the first into the second composition, at least a portion of this shear treatment preferably taking place while the composition contains 5 per cent or more moisture, and the coagulum may be pelletized while wet to cause smearing of the carbon black. The amount of carbon black within the first composition is preferably greater than that within the second composition, preferred proportions being 40-100 parts by weight and 20-35 parts by weight respectively per 100 parts of composition. A major portion of the carbon black in the first composition is preferably a channel black, while the major portion of the carbon black used in the matrix is preferably a type giving less stiffening action per volume than the carbon black of the first composition and is preferably a furnace black. The polymeric material may be natural rubber or a polymer of one or more of butadiene or butadiene compounds such as butadiene-1,3, isoprene, haloprenes such as chlor-2-butadiene-1,3, 2,3-dichlorobutadiene-1,3, dimethyl butadiene and cyano-2-butadiene-1,3. It is preferred, however, especially for the disperse phase, to use copolymers of a major proportion of one or more such conjugated dienes, particularly butadiene and/or isoprene, with a minor proportion of one or more other polymerizable unsaturated materials, especially those containing an aliphatic olefin or vinyl group including aryl vinyl compounds such as styrene, alpha-alkyl styrenes, halogen-substituted aryl vinyl compounds such as mono-, di-and tri-chlor-substituted styrenes having chlorine substituents either in the nuclear or aliphatic portion thereof and including meta-, ortho- or para-chlorostyrenes, 2,5-, 2,4- and 1,2-dichlorostyrenes, acrylic acid, ethyl acrylate, methyl acrylate, methyl methacrylate, dimethyl-vinylethinyl carbinol, vinyl pyridine and lower olefin hydrocarbons such as isobutylene. There may also be present other materials such as methacrylylurea and vinyl ketones, substituted vinyl ketones, unsaturated, low molecular weight acids, such as fumaric acid, crotonic acid, their esters and amides, with or without polyhydric alcohols, mixtures of vinyl amide and phthalic anhydride or vinyl phthalamide, to modify the properties of the polymer. There may also be used rubbery products produced by condensation such as the reaction products of dibasic acids such as sebacic acid with glycols and diamines, and vulcanizable olefin polysulphide rubbers. Preferably a substantial proportion of the first composition is a synthetic rubber while a substantial proportion of the matric composition is a natural rubber. Normal compounding ingredients such as curing agents, particularly sulphur, pigments, activators, particularly zinc oxides, plasticizers, particularly pine tar, stearic acid, antioxidants and accelerators, particularly mercaptobenzothiazole, may be incorporated. The matrix composition is preferably plasticized with a polymerizable liquid or semi-solid which is capable of being further polymerized preferably to a rubbery polymer, a preferred plasticizer being obtained by polymerizing butadiene or a mixture of butadiene and carbon black to a liquid of syrupy consistency. Specification 328,812, [Class 70], is referred to.ALSO:A heterogeneous plastic composition suitable for forming rubberlike articles is prepared by forming a first composition containing carbon black and a polymeric material, this first composition being plastic but vulcanizable to the elastic state characteristic of soft rubber, forming a second composition containing a polymeric material and also if desired containing carbon black, this second composition being of substantially greater plasticity, preferably at least 20 points as measured by the Mooney plastometer, than the first composition but also vulcanizable to the elastic state characteristic of soft rubber, and incorporating the first composition as discrete dispersed particles throughout the second composition as a matrix. Portions of the first composition may be added to a mass of the second composition while the latter is being masticated and a major proportion of the carbon black of the first composition may be incorporated therein by adding it to a latex of the polymeric material in the first composition, coagulating the latex and subjecting the coagulum to shear treatment prior to the incorporation of the first into the second composition, at least a portion of this shear treatment preferably taking place while the composition contains 5 per cent or more moisture, and the coagulum may be pelletized while wet to cause smearing of the carbon black. The amount of carbon black within the first composition is preferably greater than that within the second composition, preferred proportions being 40-100 parts by weight and 20-35 parts by weight respectively per 100 parts of composition. A major portion of the carbon black in the first composition is preferably a channel black, while the major portion of the carbon black used in the matrix is preferably a type giving less stiffening action per volume than the carbon black of the first composition and is preferably a furnace black. The polymeric material utilised may be natural rubber or a polymer of one or more of butadiene or butadiene compounds such as butadiene-1,3, isoprene, haloprenes such as chlor-2-butadiene-1,3,2-3 dichlorobutadiene 1,3, dimethyl butadiene and cyano-2-butadiene 1,3. It is preferred, however, especially for the disperse phase, to use copolymers of a major proportion of one or more such conjugated dienes, particularly butadiene and/or isoprene, with a minor proportion of one or more other polymerizable unsaturated materials, especially those containing an aliphatic olefin or vinyl group including aryl vinyl compounds such as styrene, alpha-alkyl styrenes, halogen-substituted aryl vinyl compounds such as mono-, di- and trichlor-substituted styrenes having chlorine substituents either in the nuclear or aliphatic portion thereof and including meta-, ortho-or para-chlorostyrenes, 2,5-, 2,4- and 1,2-dichlorostyrenes, acrylic acid, ethyl acrylate, methyl acrylate, methyl methacrylate, dimethyl-vinylethinylcarbinol, vinyl pyridine and lower olefin hydrocarbons such as isobutylene. There may also be present other materials such as methacrylylurea and vinyl ketones, substituted vinyl ketones, unsaturated, low molecular weight acids, such as fumaric acid, crotonic acid, their esters and amides, with or without polyhydric alcohols, mixtures of vinyl amide and phthalic anhydride or vinyl phtalamide, to modify the properties of the polymer. There may also be used rubbery products produced by condensation such as the reaction products of dibasic acids such as sebacic acid with glycols and diamines and vulcanizable olefin polysulphide rubbers. Preferably a substantial proportion of the first composition is a synthetic rubber while a substantial proportion of the matrix composition is a natural rubber. Normal compounding ingredients such as curing agents, pigments, activators, plasticizers, stearic acid, anti-oxidants and accelerators may be incorporated. The matrix composition is preferably plasticized with a polymerizable liquid or semi-solid which is capable of being further polymerized preferably to a rubbery polymer, a preferred plasticizer being obtained by polymerizing butadiene or a mixture of butadiene and carbon black to a liquid of syrupy consistency. Specification 328,812, [Class 70], is referred to.