These writings or markings are visible at a greater or less distance in proportion to the depth of the furrows. In some instances they are distinctly visible upon the rocks on the banks of the river at a distance of one hundred yards; in others they are so faint that they can only be seen in certain lights by reflected rays from their polished surfaces. They occur upon greenstone, granite, quartz-porphyry, gneiss, and jasperous sandstone, both in a vertical and horizontal position, at various elevations above the water. Sometimes they can only be seen during the dry season, when the rivers are low, as in several instances on the Berbice and Cassikytyn rivers. In one instance, on the Corentyne river, the markings on the rock are so much above the level of the river when at its greatest height, that they could only have been made by erecting a staging against the face of the rock, unless the river was at the time much above its usual level. The widths of the furrows vary from half an inch to one inch, while the depth never exceeds one-fourth of an inch. Sometimes the markings are almost level with the surrounding surfaces, owing to the waste or degradation by atmospheric influences, which have acted with greater force upon the rough rock than on the polished face of the grooved markings. The furrows present the same weather-stained aspect as the rocks upon which they are cut, and both the rocks and the furrows are in some instances coated with a thin layer of the oxides of iron and manganese.