Estimating rates of return on British overseas investments is a popular topic among recent studies on the effects of capital exports on British industrial performance since the last quarter of the 19th century. Until shares in foreign mines became attractive assets in the London capital market, domestic investments dominated, yet there are virtually no calculations on them. Based on reconstructed cash flow data collected from the Mining Journal, estimations on portfolio investment will be made using the concept of the Internal Rates of Return (IRR). They will be calculated for major Devon and Cornwall mines between 1861 and 1892 to speculate about the following arguments ; that firstly, the south-west mines, except for a few, were not considered as desirable financial assets because of their short lives and extreme fluctuations in financial performance, and secondly, that on the other hand, the fluctuations themselves created opportunities for making immediate profits by sharedealing. As such, they were very attractive as objects for speculation. There may have been two kinds of investors : those who were shareholders of a small number of celebrated mines which were continously lucrative in terms of both dividends and high evaluation in the share market, and those who were seeking quick capital gains from sharedealing and were more interested in the market price of the shares than the mining activity itself. An analysis of the ways in which developments in the share market were reflected on the conservative attitudes in mine management in the south-west of England will be required.