Since coal represents the accumulation of material on the floor of ancient swamps, the coal tends to form in layers known as seams.
(coal; represents; the accumulation of material on the floor of ancient swamps) (the coal; tends; to form in layers) (the coal; to form; L:in layers) (layers; known; as seams)
coal the coal the coal layers
represents tends to form known
the accumulation of material on the floor of ancient swamps
to form in layers
L:in layers
as seams