C L A 



[ 92] 



C L A 



of Sir Charles Lyell. The follow- 

 ing description is extracted from 

 Dr. MantelTs works. The Cla- 

 thraria Lyellii bears an analogy to 

 the yucca, and dracsena or dragon- 

 blood plant. Stems, with the 

 markings of the bases of the 

 leaves, point out the relation of 

 this vegetable to the arborescent 

 ferns, while its internal structure 

 is essentially different. The cla- 

 thraria has only been found in the 

 quarries in Tilgate Forest. This 

 vegetable appears to have possessed 

 a thick epidermis, or false bark, 

 formed by the union of the bases 

 of the leaves, and covered external- 

 ly with distinct rhomboidal scales, 

 each scale being surrounded by an 

 elevated ridge. The form of the 

 leaves is not positively known, 

 although, from some imperfect 

 traces on the stone in a specimen 

 bearing the impressions of the 

 cicatrices of the bases of the leaves, 

 there is reason to conclude that 

 they were of a lineari-lance olate 

 form. The axis, or interior part 

 of the trunk, originally enclosed by 

 the bark, occurs in the state of 

 solid subcylindrical blocks of sand- 

 stone, attenuated at their base, the 

 surfaces of which are marked with 

 longitudinal interrupted ridges, and, 

 in some instances, are deeply 

 imbricated ; they are generally of 

 a dark-brown colour. 



CLAVAGE'LLA. A genus of bivalves, 

 of which only one species has been 

 found recent, in the Sicilian seas. 

 It has two irregular, flattish valves, 

 one of which is clasped by the 

 tube, the other being left free. 

 Mr. Sowerby observes, " The shells 

 composing this genus are found in 

 stones, madrepores, &c. and appear 

 to form the connecting link between 

 Aspergillum, which has both valves 

 cemented into the tube, and Fistu- 

 lana, which has both free." 



CLA'VICLE. (clavicula, Lat. clavicule, 

 "Fr. clavicula, It.) The collar-bone. 



CLAY. When clay is quite pure and 

 unmixed (and in this state it is one 

 of the rarest substances in the 

 mineral kingdom) it is termed 

 alumina, but under the term clay 

 is comprehended anxtensive class 

 of compounds, of which silex is a 

 principal constituent. Clay, then, 

 may be defined an unctuous and 

 tenacious earth, capable of being 

 moulded into form ; any earth 

 which possesses sufficient ductility, 

 when kneaded with water, to be 

 fashioned like paste, by the hand, 

 or by the potter's lathe. Clays 

 are firmly coherent, weighty, com- 

 pact, and hard when dry, but stiff, 

 viscid, and ductile when moist ; 

 being smooth also and unctuous to 

 the touch. Besides alumina and 

 silica, clays often contain carbonate 

 of lime, magnesia, barytes, oxide 

 of iron, &c. When clay is breathed 

 on, it yields a peculiar smell ; it 

 has also a strong affinity for 

 moisture, which is shown by its 

 sticking to the tongue, when ap- 

 plied to it. The purest clay is 

 kaolin, or porcelain clay. All clays 

 appear to be mechanical deposits, 

 not one of them occurring crystal- 

 lized, or with a crystalline structure; 

 some are found slaty. 



CLAY-SLATE. (The argillite of Kir- 

 wan.) An indurated clay or shale, 

 common to the fossiliferous and 

 metamorphic series. Clay-slate is 

 opaque, of various shades of colour, 

 and of different degress of hardness, 

 but easily scratched by iron. It is 

 composed of about fifty per cent. 

 of silex, twenty-five of alumine, 

 and ten or twelve of oxide of iron. 

 Some varieties are used as whet- 

 stones. The common hone is a 

 variety of slate containing a smaller 

 proportion of alumine and some 

 lime. 



CLA'YSTONE. An earthy stone re- 

 sembling indurated clay, and gene- 

 rally of a colour approaching to 

 purple ; it is a variety of prismatic 

 felspar. 



