In all shaft mines the openings to the surface hereinbefore provided for shall be separated from each other by natural strata at all points by a distance of not less than one hundred fifty (150) feet, and at all mines worked by slope openings, the distance separating openings shall not be less than fifty (50) feet of natural strata: Provided, that in any mine opened heretofore the distance between said openings may be less, if the mine inspector of the district shall deem it impracticable to comply with the foregoing requirements. Where the two openings shall not have been provided as hereinbefore required, the mine inspector shall cause the second opening to be made in every mine without delay and in no case shall furnace ventilation be used where there is only one opening into the mine.