The practice at Thetford is to close down entirely for the winter months, it not having yet been found advantageous, in view of the limited market, coupled with the difficulty of outdoor quarrywork, to encounter the extra expense of working at this season. Some of the owners, Messrs. King in especial, being largely interested in the lumber trade, to which they devote themselves in the winter, are able to find plenty of employment at that season for their men and others, who then go off into the woods. At Thetford, as elsewhere, great mistakes, arising mainly from inexperience and want of the knowledge now possessed, were made when the mines were first opened up, which will inevitably entail serious loss in the future; as an instance, much valuable ground is now seen to be covered up by the dumps, which will unquestionably have to be moved presently when the land is wanted for working. The same want of foresight, for which there was less excuse with the Thetford experience to guide them, will inevitably cause trouble in the near future at Black Lake, on the property of the Anglo-Canadian Company, even to a more serious extent. The parties who first opened up the ground here evidently knew what they were about, but their immediate successors, being destitute of all practical mining knowledge, have, by their neglect of professional assistance, committed such errors of judgment as will presently occasion very serious expense to the company. At their main pit many thousands of tons of waste rock have been dumped on to some of the richest part of the ground, and this must be again moved before that ground can be worked. From the peculiarity of the work, there probably exists no class of mining which so absolutely necessitates the services of a practical mining engineer, in marking out the land in the first instance for mining and dumping, as that for asbestos does. The cost of extraction varies in different localities, depending mainly on the mass of barren rock to be encountered and removed. At Thetford the cost may be put at from $20 to $25 per ton, the latter probably being nearer the average. On the Anglo-Canadian Company's property at Black Lake it is a more serious matter. There the quantity of barren overlying rock and earth is enormous, and detracts immensely from the value of the mines. The minimum cost here is $28. In addition to asbestos, it is worthy of note that the whole of the Thetford district is rich in minerals. Among others some large and valuable deposits of chromic iron are found in the immediate neighbourhood, within but a short distance of the asbestos mines. THE COLERAINE GROUP. Four miles farther down the line of railway from Thetford we come to the Coleraine group of asbestos mines, situate at Black Lake.