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.]
This report sets forth, that many granite quarries were found in activity at Aberdeen, some of which were capable of producing larger blocks of stone than are usually met with, but that still it was doubtful, whether any single quarry would be found to produce a sufficient number of large blocks for this work in any reasonable time. Upon the quality of the stones respectively; the report states, that “the granite of Aberdeen is very strong and durable in its nature, and having been used in works where the sea has acted upon it for time immemorial, no doubt can possibly be entertained as to its adaptation to a work of this kind. There is also every reason to believe that the Mylnefield stone resists the sea and weather equally well, but we have not been able to collect such positive proof of this as of the other; for, although a great number of Mylnefield stones have been used in the piers of the harbour of Dundee, yet, as these works consist of stones from other quarries, having the same appearance, and nearly the same composition, there is no possibility of our saying whether some of the stones that appear in a wasting state, may not have been from that quarry, although we have great reason to believe they have not. However, where facts cannot be positively ascertained doubts exist, and we think that a Light-house upon the Bell Rock is too important a work to permit the leaving of the slightest doubt about the durability of the materials. We have, therefore, no hesitation in recommending that the outer part of the building, at least as high as the first apartment, should be of granite; and as this is the great bulk of the work, it may be as well to complete the outer course of granite.”