From the abundance of clay upon its site, London is, as might be expected, a brick-built city; although the ingenuity of our age has cased miles of streets with cement, to imitate stone. This prevalence of clay is, in great measure, explanatory of the vastness of the metropolis. It is nowhere better illustrated than in the fact of "the Five Fields," (between Pimlico and Chelsea,) formerly a clayey swamp, being now the site of some of the finest mansions in London. A few years ago, the clay retained so much water that no one would build there, and "the Fields" were the terror of foot-passengers proceeding from Westminster to Chelsea after nightfall. At length, Mr. Cubitt, on examining the strata, found them to consist of clay and gravel, of inconsiderable depth. The clay he removed, and burned into bricks; and by building upon the substratum of gravel, he converted this spot from the most unhealthy to one of the most healthy, to the immense advantage of the ground landlord and the whole metropolis. This is one of the most perfect adaptations of the means to the end, to be found in the records of the building art.