The assessor, after such statement has been rendered, shall have the right to examine the books and accounts of any person, corporation or association engaged in mining as mentioned in this chapter, in order to verify the statement made by such person, corporation or association, and if from such examination he finds such statement false, he must assess the net proceeds in the same manner as if no statement had been made and delivered, by making an estimate from the best sources within his reach, and if satisfied that the false statement was intentionally so made, he shall add as a penalty therefor, to the amount of the net proceeds so found, fifty per cent (50%) thereof, which amount thus increased shall constitute the sum upon which the taxes must be levied and collected, and such assessment shall be binding, effectual and lawful, and the value so fixed by the assessor shall not be reduced by the county board of equalization.