The bulk and weight of coal required in the majority of manufacturing industries is large in comparison with the bulk and weight of other raw materials. This is not always true -- as with the manufacture of pig iron and steel from low-grade iron ores -- but it did lead to the concentration of industrial developments on the coal-fields, a phenomenon well seen in such countries as Britain where the Industrial Revolution came before the days of oil or electricity. Coal being a solid must be distributed mainly by rail or water.
I would think "a phenomenon" is the appositive of "the concentration of industrial developments on the coal-fields." Thus, "it" would refer to "this" in the previous sentence, which is the statement point in the first sentence. Does it make sense?
dummy-pronounstag. ) – Jason Bassford May 24 '19 at 12:50