Complex sentence: The lower mountains in the extreme west are wooded, but the extent of forest declines eastwards, and the eastern Pyrenees are peculiarly wild and barren, all the more since it is in this part of the chain that granitic masses prevail.
Split into simpler sentences: The lower mountains in the extreme west are wooded, but the extent of forest declines as one moves eastwards. The eastern Pyrenees are peculiarly wild and barren, all the more since it is in this part of the chain that granitic masses prevail.