Sands of different physical characteristics were cemented with quartz in hydrothermal reactors at temperatures from 255 degrees C to 360 degrees C and pressures from 2000 psi to 11,000 psi. Relatively high thermal gradients were employed to produce the circulation of solution through the sands. Weak solutions of NaOH, Na 2 CO 3 or K 2 CO 3 were used. With freely circulating solutions, well sorted coarse sands became cemented faster than fine-grained sands because of the greater influx of cement into the more permeable coarse sand. However, finer grained sand samples were cemented much more rapidly than coarser sands where influx of cementing solution was the same in both cases. The rate of cementation appeared to be a function of the surface area of the grains. Cementation proceeded considerably faster in highly angular sands than in rounded sands of the same grade size. Although the angular sand had a higher porosity initially, its porosity became less than that of round sand after a moderate amount of cementation. Polycrystalline aggregates of quartzite or chert grew much more slowly than grains of single crystals with rate of growth decreasing with decreasing size of crystals comprising the aggregates. In impure mixtures, the degree of cementation was proportional to quartz content. Microcrystalline cement formed with rapid introduction of concentrated cementing solutions. Commonly, normal overgrowths formed on the grains near the ends of the c-axes while microcrystalline quartz was simultaneously deposited on other parts of the grains. Large amounts of cement in the form of normal overgrowths imparted little strength to sands, especially in the coarse types. Sands of polycrystalline grains developed good cohesion because of interlocking of the diversely oriented growths on these grains. A relatively small amount of microcrystalline cement imparted considerable strength to the sands.