1. This layer of rock contains a lot of flint.
    这一岩层中有大量燧石.

2. A layer of adipose tissue under the skin
    皮下的一层脂肪组织.

3. If there had not been a hard layer of rock beneath the soil, they would have completed the job in a few hours.
    如果不是因为土壤下面有一层坚硬的岩石，他们的营救工作仅用几个小时就可以完成了。

4. From June to September the Nile rises and submerges the whole valley; then in the beginning of October it begins to fall, leaving the land saturated with water and covered with a layer of rich, black soil.
    从六月到九月，河水暴涨，淹没了整个流域；到十月初，河水开始减退，剩下饱含大量水分的土地，还覆盖着一层肥沃的黑土。