PDF MISCELLANEOUS NONMETALLIC MINERALS. Publication 21. Geological Series ...
Prior to 1903, less than 10 per cent of the coal was mined by coal cutting machines. Five years later, the percentage had increased to per cent, in 1910 per cent respectively. ... more efficient types of the short wall coal cutting machines. Publication 21, Geological Series 17 / Part II Miscellaneous NonMetallic Minerals Page 2 ...