Complex sentence: From around 1830 geologists were beginning to date rocks according to the embedded fossils and had formulated the law of superposition whereby younger rocks lie above older ones but recognising that some layers may be missing because they had been eroded away.
Split into simpler sentences: From around 1830 geologists were beginning to date rocks according to the embedded fossils. The law of superposition whereby younger rocks lie above older ones was very well established and it was recognised that some layers may be missing because they had been eroded away.