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| | On the Formation of Coal (1870) |
 | | The purer coals certainly consist principally of cubical tissues with some true woody matter, and the spore cases, andc., are chiefly in the coarse and shaly layers. |
 | | Coal, such as that which has been described, is [147] always found in sheets, or "seams," varying from a fraction of an inch to many feet in thickness, enclosed in the substance of the earth at very various depths, between beds of rock of different kinds. |
 | | What are now known [154] as coal districts owe their importance to the fact that they were areas of slow depression, during a greater or less portion of the carboniferous epoch; and that, in virtue of this circumstance, Mother Earth was enabled to cover up her vegetable treasures, and preserve them from destruction. |
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