Complex sentence: They comprise the Carboniferous Limestone, the overlying Gritstone and the Coal Measures which occur only on the extreme margins of the area and infrequent outcrops of igneous rocks including lavas, tuffs and volcanic vent agglomerates.
Split into simpler sentences: They comprise the Carboniferous Limestone, the overlying Gritstone and finally the Coal Measures, though the latter occur only on the extreme margins of the area. In addition there are infrequent outcrops of igneous rocks including lavas, tuffs and volcanic vent agglomerates.