underclays

The peat bog is a great mass of vegetable matter, which is every year growing thicker and thicker; and underneath it there is almost always a bed of thin clay, in look very much like the underclays, and this thin clay is penetrated by the rootlets of the moss forming the peat, exactly the same way as the underclays of the coal measures are penetrated by the stigmaria and its rootlets.

Scientific American Supplement, No. 447, July 26, 1884 Various

But all underclays agree in two points: they are all unstratified.

Scientific American Supplement, No. 447, July 26, 1884 Various

This theory was for some time but poorly received; but after the discovery of Sir William Logan, that every bed of coal had a bed of underclay beneath, and the discovery of Mr. Binney, that these underclays were true soils on which plants had undoubtedly grown, there was no doubt whatever that this was the real and true explanation of the matter.

Scientific American Supplement, No. 447, July 26, 1884 Various

Such underclays withstand intense heat and are used in making fire brick, because their alkalies have been removed by the long-continued growth of vegetation.

The Elements of Geology William Harmon Norton 1900

Infusoria, possible occurrence in underclays of coal.

More Letters of Charles Darwin — Volume 2 Charles Darwin 1845

- Darwin's wish that he should examine underclays.

More Letters of Charles Darwin — Volume 2 Charles Darwin 1845