From this state of the mineral strata, it naturally follows, that those who inhabit the sandstone districts employ that beautiful, easily worked, and, in many instances, highly durable stone, in architecture; and so of the other districts, according to the predominating species of their stone. For a building, therefore, in a country situate like that of the Bell Rock, abounding with sandstone of the first quality, this description of stone obviously presented itself, both as the most accessible and economical. But when the importance of this work came to be fully considered in all its relations, a little additional expence was not to be allowed to regulate a point so essential, without a due regard to what might ultimately prove the most durable and permanent fabric. [Sidenote: The use of granite and sandstone is resolved upon.] The attention of the Commissioners was consequently directed to the use of granite, as combining the greatest number of properties for such a building. Some doubts, however, having existed, as to the certainty of procuring blocks of that stone of sufficient dimensions, it became a matter of importance to determine this point, and also to ascertain the quality of the sandstone, of which it had been proposed to form at least the hearting of the solid part. The Commissioners, therefore, in the month of November 1806, required a special opinion from Mr Rennie and the writer upon this subject; who accordingly visited the sandstone quarry of Mylnefield near Dundee, and the granite quarries in the neighbourhood of Aberdeen, and made a report to the Board, which is given in the Appendix, No. IV. [Sidenote: Report of Mr Rennie and Mr Stevenson.]