It would seem that, from the close of the seventh century, a wave of mining industry swept over Japan. Silver was procured from the provinces of Iyo and Kii; copper from Inaba and Suo, and tin from Ise, Tamba, and Iyo. All this happened between the years 690 and 708, but the discovery of copper in the latter year in Chichibu was on comparatively the largest scale, and may be said to have given the first really substantial impetus to coining. For some unrecorded reason silver pieces were struck first and were followed by copper a few months later. Both were of precisely the same form--round with a square hole in the middle to facilitate threading on a string--both were of the same denomination (one won), and both bore the same superscription (Wado Kaiho, or "opening treasure of refined copper"), the shape, the denomination, and the legend being taken from a coin of the Tang dynasty struck eighty-eight years previously. It was ordered that in using these pieces silver should be paid in the case of sums of or above four mon, and copper in the case of sums of or below three won, the value of the silver coin being four times that of the copper. But the silver tokens soon ceased to be current and copper mainly occupied the field, a position which it held for 250 years, from 708 to 958. During that interval, twelve forms of sen* were struck. They deteriorated steadily in quality, owing to growing scarcity of the supply of copper; and, partly to compensate for the increased cost of the metal, partly to minister to official greed, the new issues were declared, on several occasions, to have a value ten times as great as their immediate predecessors. Concerning that value, the annals state that in 711 the purchasing power of the mon (i.e., of the one-sen token) was sixty go of rice, and as the daily ration for a full-grown man is five go, it follows that one sen originally sufficed for twelve days' sustenance.** *The ideograph sen signified originally a "fountain," and its employment to designate a coin seems to have been suggested by an idea analogous to that underlying the English word "currency." **"At the present time the wages of a carpenter are almost a yen a day. Now the yen is equal to 1000 mon of the smaller sen and to 500 mon of the larger ones, so that he could have provided himself with rice, if we count only 500 mon to the yen, for sixteen years on the wages which he receives for one day's labour in 1900." (Munro's Coins of Japan.)