Shales are a type of sedimentary rock that is composed primarily of clay-sized minerals. When looking for synonyms for the word "shales," there are a few options to choose from. One alternative is to use the term "mudstone," which is very similar in composition to shale but has less fissility. Another option is to use the term "argillite," which is a harder and more metamorphosed version of shale. Finally, the term "claystone" can also be used, which is a type of sedimentary rock that is composed primarily of clay minerals, but without the laminae that give shale its characteristic fissility. Regardless of which term is used, all of these synonyms for shale refer to rocks that were formed through the deposition of sediment in a sedimentary basin.