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【geology】地質學：mesosphere layer　地球內部結構 
　
1. crust [krʌst] n. 地殼 (continental/oceanic crust 大陸/海洋地殼)
　The earth's crust is its outer layer. The thickness of the earth's crust varies from about 5 miles under the oceans to about 25 miles under the continents.
　
2. stratum [ˋstretəm] n. 地層 smile 表情符號 layer; pl. strata)
　Strata contain clues that tell geologists what the earth was like in the past. The crust is the layer that we live on.