384

HISTORY OF ENGINEERING.

Book I.

At the Bcare quarries, on the sea coast, near the south-east corner of Devonshire,Smeaton found the bed of freestone was of considerable thickness, and with so great a coverof earth upon it, that it wasworked in underground ca-vities, the superincumbentearth being supported bypillars, formed of the de-tached parts of the stratumleft standing. This calca-reous stone resembled theBath, but its bed was sothick, that blocks of anysize might be drawn from it.

It is very compact, free fromfissures, and yet so soft thatit can be cut with the com-mon saw ; after exposure itbecomes hard, and in thebuildings at Beare, where itis used, is covered withmossy coating, and remainsperfectly sound and freefrom decay.

Sand and limestone haveeach their peculiar lichen,and their respective qualitiesmay be known by a carefulexamination of the plantsproduced upon their sur-faces : granite and gneissrocks are often found co-vered with Lecanora gelida,

Lecidea petraa, Silacea, La-picida, Verrucaria glaucina,

&c. The rock crystalknown by its being the ha-bitat of Lecanora fusco-atravar. dendricata. The rocksof porphyry exhibit on theirsurface the Lecidea pustu-lata and confluens, Parmeliaciiiaris and furfuracea, andthe Gyrophora deusta. Themica slate rocks are foundcovered with Corniculariatristis and exilis, Gyrophorapolyphylla, and of the va-riety frigida of Lecanoratartarea. The clay slateabounds with such lichensas Lecidea cupularis, Leca-nora decipiens, &c. Sand-stone near the sea is fre-quently found covered withthe Ramalina scopulorum,

Lecanora parella and atra.

Limestone is furnished withthe Lecidea immersa, Col-lema nigra, Verrucaria rau-ralis, Urecolaria calcarea,and Thelotrema exanthe-matica. And any one ac-quainted with the charac-teristics of the several lichensmay at once distinguish thevariety of stone used inbuilding, where, in the

course of time, they generally are covered with specimens of one kind or the other.

ELEVATION OF EDDVSTONE L1GHTHOUSB.