1·The gravel frequently is sieved and graded sand that is packed in bags.
砾石通常是经过筛析和粒度分级的袋装砂子。
2·Many patient taxonomists from several European countries and America then sieved through litres of muddy gravel with microscopes and pairs of tweezers to see what treasures had been captured.
来自欧洲几个国家和美国的许多分类学家耐心地用显微镜和镊子对数以升计、沾满烂泥的沙砾进行筛分，来看看什么样的宝贝被铲了上来。