Akind of sedimentary rocks formed from plant merrill over a long period of time is a

The answer; coal

This rock is therefore mainly made of carbon, hydrogen, oxygen, nitrogen, and sulfur compounds just like plants. It begins with the formation of peat when dead plants accumulate in waterlogged soils and the plants decompose partially as they are submerged in more sediments. Over many years and more sediments, the accumulated pressure and temperature causes the metamorphoses of the peat into coal.  

it would be rhyolitic hope this