这是the more...the more句型
darker and more,the closer they went.darker and more=the darker 吗？感觉这里的语言是非正式的语言(口语化)，属于想到哪儿说到哪儿。到darker and more其实已经说完了，又想到该补充一句它的原因（即 the closer they went). The closer they went, the darker and the more (ash and bits of rock were falling onto the ship).