Physical differences between water-saturated cohesionless sand and cohesive fine mud account for many sedimentary structures. Relative bulk cohesion is controlled by relations of intergranular adhesion caused by electromagnetic or Van der Waal's forces versus the disturbing effect of forcible migration of pore fluid or gas. Where expulsion of fluid is slow, mud retains greater cohesion than sand. If sand is loosely packed, movement of fluid, especially if sudden, may cause spontaneous liquefaction and conversion to a viscous slurry with negligible strength.

Empirical evidence of major differences of cohesion comes from abundance of shale pebbles, load structures, current sole marks, contorted stratification, and clastic intrusions. These are most characteristic of alternating sand-mud sequences which possessed many inherent-strength discontinuities. All degrees of liquefaction, flow, intrusion, and stopping are "frozen" in rocks--from local in-place liquefaction of sand strata to immense dike complexes and sand volcanoes. Clastic dikes are of special interest. Typically they contain fine sand, but in a few places may have coarse gravel. Wall rock is generally mud, and stoped cohesive mud xenoliths are common; some dikes split and even rejoin along strike. Internal lamination and some grain alignment may occur, and these suggest chiefly l minar flow of viscous suspensions. A few dikes display excellent groove and flute marks on their sides, evidencing scouring of cohesive wall rock during intrusion and indicating direction of intrusion. Some large dikes completely evacuated their source stratum with concomitant subsidence of all overburden.

Important relative age criteria are provided. Most dikes are straight, suggesting that compaction was completed largely before intrusion. In fact, compaction

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of mud probably provided most of the pore water which liquefied intercalated sand. The exceptional wavy dikes apparently were intruded earlier and compacted later with their wall rock. Sill-like masses are more problematical; some evidence clearly indicates liquefaction after burial, but others of these masses could have formed at the depositional interface. Certain strata were liquefied in place after burial and are not true intrusions, although they strongly resemble sills. Some synsedimentary folds can be distinguished from tectonic ones where sandstone dikes cut through them and prove an early, soft-sediment origin. Dikes along slaty cleavage have been cited as evidence of early formation of such cleavage. Some intrusions are useful in determining dates of migration and deposition of ores or fluids. Dikes may show close relations with regional structures, but many do not. Though more common in tectonically mobile regions, they also occur in stable ones. They most probably originated by shocks from earthquakes or from sudden loading. They are more common than is generally realized and their usefulness to the geologist has not been appreciated fully.

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