hard coal and hard roof
坚硬的煤和坚硬的屋顶
以上为机器翻译结果，长、整句建议使用 人工翻译 。
The hard-and-thick strata ACTS both as the floor of coal seam No. 7 and as the roof of coal seam No. 9 in Zhangshuanglou coal Mine; and coal seam No. 9 is also hard.
张双楼煤矿主采煤层为7煤和9煤，两层煤之间赋存唯一一层厚硬岩层，即9煤层的直接顶也是7煤层的直接底。
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