Laboratory studies of clay-water systems and field observations of the properties of argillaceous rocks of the Paleozoic systems in the Illinois Basin suggest a close relationship between the environments of deposition of such rocks and the origin of such internal structures such as fissures, slickensides, fissility, and synaeresis cracks. Therefore, the purpose of this report is to suggest a possible origin for fissility, slickensides, synaeresis cracks and fissures. Clay minerals in fissile argillaceous rocks, such as shales, are oriented with the longest dimensions more or less parallel to the bedding. The shales generally contain few slickensides, are relatively resistant to disaggregation in water, and generally contain the better-developed crystalline clay minerals. In contrast, the clay minerals have random orientation in shales that show poor or no fissility. Such rocks commonly contain an abundance of slickensides, break down readily in water, and generally contain a larger percentage of mixed-lattice clay minerals than the fissile shales.