AND PORTO SANTO. 85 



paratively safe descent from the south-western end, but where 

 many of the strata were covered by debris. We first ascended 

 about 100 feet of the same tufa which I have described at Porto 

 Santo ; then twelve feet of Hmestone, of a gi'anular sandy struc- 

 ture, glimmering lustre, and emitting an alliaceous odour when 

 struck ; it contained no fossils, or at least none that I could dis- 

 cover, after examining and breaking away its surface in various 

 directions, and after spHtting numerous large fragments' ; it is of a 

 buff ground, sprinkled with grey and red spots, or grains. Above 

 this, I found about fifty feet of a conglomerate of nodules of basalt, 

 or rather of wakke (from its colour, fracture, and specific gravity); 

 these nodules were very large below, but diminished in size 

 upwards, until they looked like rusty nail-heads, inserted in a 

 gi'ound of a ferruginous sandy earth, not effervescing, of a brick 

 and dull orange red, and yielding to the nail : this earth was 

 covered with connected lines (spread hke a net-work) of mam- 

 miUated carbonate of lime, pervading the whole mass, some- 

 times lining small cavities, and of a dull white passing into 

 grey. We then cHmbed over from eight to ten feet of a con- 

 glomerate limestone, generally of a chalky white, soiling the fin- 

 gers, sometimes of a whitish blue, and containing large nodules 

 of wakke, and imbedded masses of a granular sandy hmestone, 

 resembhng that before described, but with more grey, and less red 

 in its composition ; the additional grey appearing to have been 

 supphed by very small fragments of basalt. The white part 

 of this limestone presented immense masses of Lamarck's cate- 

 niporce (tubipora catemdata Lin. Gm.), and with much difficulty 

 I chiselled out (for like that below, it was excessively hard) some 

 perfect moulds of a large cardium ; and the cardium edule ; one 



* The fetid limestone, submitted to an excess of muriatic acid, affords a residue of 

 nearly .49 of silex ; the white limestone leaves scarcely any, and its specific gravity 

 24, is 1 less than that of the fetid. 



