The grains of sand may be broken and become angular during their long trip to the ocean, or they may become rounded by rubbing against each other. If they exist in sand dunes, blown about by the wind before being cemented into rock, the grains usually become somewhat rounded. Even after sandstones are buried beneath other rocks, silica, which is carried in solution by ground waters percolating through the sandstone, may crystallize out on the sand grains and restore some brilliant, angular crystal faces to the otherwise rounded grains. Cementation of loose sand to more or less firm sandstone is due to the presence of clay, iron oxides, or calcite (mineral of limestone) which may be deposited with the sand. All of these cements are softer and weaker than quartz, thereby being broken first and freeing the harder quartz when the rock is scratched or crushed. A variety of very hard sandstone called quartzite is one that is so strongly cemented that it breaks through the sand grains instead of around them as is the case with ordinary sandstone. This condition is brought about by their being cemented with silica (chemically the same as quartz), which makes for essentially uniform hardness throughout the rock. Quartzites are, as previously noted, extremely hard, and resist abrasion and chemical weathering. Reddish quartzite boulders occur rather abundantly north of the Missouri River in the glacial clay, sand, and gravel which overlie the sedimentary rocks that form the bed rock or country rock there. Locally, the hard, red quartzite boulders may be called “red niggerheads”, although the term “niggerhead” is more often applied to black or dark greenish black boulders of basalt (see page 48) also present in the glacial drift. It is to be recalled that the distinguishing hardness of quartzite is due to the hardness of the quartz grains plus the equal hardness of the silica cement. Asphaltic sandstone is a sandstone impregnated with a bituminous residue from the evaporation of petroleum which once occupied the pores of the rock. It has been reported from more than a dozen counties in western Missouri, but the most extensive deposits are probably in Barton, Vernon, and Lafayette counties. Attention has been directed to the origin of sandstones from ocean deposits of sand and from sand dunes, but it should be recognized also that river channels and stream valleys which contain deposits of sand (such as those on floodplains, river bottoms, and sand bars) may be covered, and the sand consolidated to sandstone. Many years ago, even long ago geologically, a large river, almost comparable in size to the Missouri river, occupied a channel which is now represented by a long narrow sandstone deposit extending from a little north of Clinton through Warrensburg to Lexington and then east through Moberly almost to Paris. Smaller channel sandstones are abundant in other areas in Missouri.