Sentence:  From the above account it will be seen how closely the <WSD>Millstone</WSD> Grit and the Lower Coal Measures are like one another in their lithological character. Each is a group of thick sandstones parted by shales, and in each beds of coal are very generally found on the top of the sandstones. The differences are these. Among the <WSD>Millstones</WSD> the sandstones are thicker, fewer, further apart, more constant both in size and character, and as a rule coarse grits or conglomerates. 
Meaning of the word marked with <WSD> tags:  A coarse-grained sandstone used for making such stones; millstone grit.