A process of formation of a casing in boreholes in additive manner, specifically in thermal drilling in geological formations, wherein - a vaporized rock and thermal energy inserted into the vaporized rock are used so that by a heat treatment a mixture of rock vapours passes through phase transformations, specifically to a liquid phase and a partially solid phase, - the heat treatment is a cooling in which the generated hot gas mixtures are cooled by coolant, by their cooling a phase transition occurs and by condensation liquid particles of rock are formed and by solidification solid particles of rock are formed in flowing mixture of rock material and cooling gases, - the generated hot gas mixtures are divided by fluid-mechanical treatment into at least two main streams, at least one stream is a stream of cold materials and at least one stream is a stream of hot materials, and at least from one of these streams other sidestreams are further diverging by which excess mixtures of rock materials and cooling gases are led away to the waste, wherein a part of the rock material from which the casing will be formed remains in the stream of cold and hot materials, - at least one stream of hot materials and at least one stream of cold materials are formed by heat treatment which is a multistage cooling of rock materials by controlled heat transformation, wherein at least two main streams are cooled to different temperature so that the stream of cold rock materials is cooled to the temperature at which gas rock material is transformed into the solid phase and solidifying rock particles are formed, which solidify before their application as the casing layer, and a stream of hot rock materials is cooled to the temperature which is above the temperature of melting and which allows them to be mixed with cold materials, thereby the formed mixture is cooled to the temperature below the solidification point and thus it forms a firm continuous casing layer, - transport of the rock material from a source of generation of hot gas mixtures, specifically mixtures of rock vapours and coolantvapours, to an area of formation of the casing by directing and application of the treated rock material to the borehole wall or to the underlying layers of the casing being formed, in the area of the casing formation, and by cooling the rock material, the layered casing is formed, wherein the casing layers formation is continuous, - by further fluid-mechanical treatment, specifically by mixing of cold rock particles and hot rock particles, a mixture of rock material is formed, which is applied to the borehole wall, wherein it forms the first casing layer, and then it is applied to the underlying layers of the moulded casing and forms additional layers. - a vaporized rock and thermal energy inserted into the vaporized rock are used so that by a heat treatment a mixture of rock vapours passes through phase transformations, specifically to a liquid phase and a partially solid phase, - the heat treatment is a cooling in which the generated hot gas mixtures are cooled by coolant, by their cooling a phase transition occurs and by condensation liquid particles of rock are formed and by solidification solid particles of rock are formed in flowing mixture of rock material and cooling gases, - the generated hot gas mixtures are divided by fluid-mechanical treatment into at least two main streams, at least one stream is a stream of cold materials and at least one stream is a stream of hot materials, and at least from one of these streams other sidestreams are further diverging by which excess mixtures of rock materials and cooling gases are led away to the waste, wherein a part of the rock material from which the casing will be formed remains in the stream of cold and hot materials, - at least one stream of hot materials and at least one stream of cold materials are formed by heat treatment which is a multistage cooling of rock materials by controlled heat transformation, wherein at least two main streams are cooled to different temperature so that the stream of cold rock materials is cooled to the temperature at which gas rock material is transformed into the solid phase and solidifying rock particles are formed, which solidify before their application as the casing layer, and a stream of hot rock materials is cooled to the temperature which is above the temperature of melting and which allows them to be mixed with cold materials, thereby the formed mixture is cooled to the temperature below the solidification point and thus it forms a firm continuous casing layer, - transport of the rock material from a source of generation of hot gas mixtures, specifically mixtures of rock vapours and coolantvapours, to an area of formation of the casing by directing and application of the treated rock material to the borehole wall or to the underlying layers of the casing being formed, in the area of the casing formation, and by cooling the rock material, the layered casing is formed, wherein the casing layers formation is continuous, - by further fluid-mechanical treatment, specifically by mixing of cold rock particles and hot rock particles, a mixture of rock material is formed, which is applied to the borehole wall, wherein it forms the first casing layer, and then it is applied to the underlying layers of the moulded casing and forms additional layers.