Samples from wells in western West Virginia and southern Ohio indicate that the Mt. Simon (Cambrian) and Rose Run (Cambro-Ordovician) sandstones are well sorted and well rounded. Most of the sands are feldspathic with some units being arkosic. Carbonate generally in the form of dolomite is common in some beds. Most of the sands were porous initially, but porosity has been reduced by a number of postdepositional processes. Feldspar cement typically ranged from 3 to 9% and significantly reduced porosity in many samples. Quartz cement was much less common ranging generally from I to 3%, although a few samples contained as much as 18% secondary quartz. In the Rose Run sandstone quartz cementation was inhibited by coatings of illite, but feldspar was able to grow mainly by pushing the illite aside. The highly calcareous sands typically had low porosity. Anhydrite was a minor pore filling in many sands but locally was abundant and reduced porosity considerably. In spite of the considerable depth of burial of some of these sands (16,000 ft.), pressure solution effects are not abnormally high. Illite promoted pressure solution and caused the elimination of porosity in some layers. Dolomite fills some pores, but much of the dolomite replaced other minerals so was not a factor in pore reduction in these cases. Secondary porosity resulted from the dolomitization of original calcite in some places. Porosity was also increased by the solution of feldspar in the feldspathic beds of the Mt. Simon Sandstone. In future exploration, the best porosity in the Mt. Simon Sandstone is to be expected in the quartzose and arkosic sands which are low in carbonate. In the Rose Run sandstone, the highest porosity is likely in the low carbonate sands where illite is sufficient to block quartz growth but not abundant enough to promote pressure solution. Solution of feldspar and dolomitization of original calcite are also favorable factors.