All fossils should occur in sedimentary rocks being abundant in limestone and limy shale but rare in sandstone. Fossils never occur in igneous rocks except when volcanic ash falls, or nearly cooled lava have overcome plants and animals. In Metamorphic rocks they are also absent except when these rocks were originally fossiliferous and subjected to very low grades of metamorphism.
In Nature fossils are found scattered in the rocks, in some cases they are accumulated in layers or patches. Those accumulated in layers or beds are called Biostroms whereas those accumulated without any distinctive layering are called Bioherms.
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