Mr. Alison on the Geology of Teneriffe. 433 



of the atomic construction of each substance is necessarily 

 presupposed : and hence this scale is addressed less to the 

 manufacturer or learner, than to the practical chemist. 

 I am, Gentlemen, yours, &c. 



John Prideaux. 

 P.S. — In several scales that have fallen in my way, the 

 stretching of the paper, in pasting, has produced important 

 deviations from the original impression. A simple contrivance 

 remedies this. A box tablet, the full length of the slider, and 

 grooved to fit, is correspondingly graduated on each side of 

 the groove. The slider is placed in this, to receive the gra- 

 duated paper; which being pasted, is allowed to remain until 

 sufficiently elongated by the humidity. It is then fixed ; and 

 such of the graduations as are out of place, by the irregular 

 stretching of the paper, are rectified by pressure with the nail. 

 The slider being now put in its own place, the side pieces, 

 having been pasted at the same time, will be found nearly to 

 correspond, and may be further stretched or shortened, and 

 any irregularities adjusted, on comparison with an uncut im- 

 pression, by the action of the nail, as before. 



LXVI. Narrative of an Excursion to the Summit of the Peak 

 of Teneriffe on the 23rd and 2i4h of February 1829: With 

 some Remarks on the Geology of that Island. By Robert 

 Edward Alison, Esq. 



[Concluded from p. 251.] 



On the Geology of Teneriffe. 



A T every step we take in Teneriffe, unequivocal marks ap- 

 ^*- pear of the great revolutions that have taken place upon 

 its surface by volcanic action: such as craters of enormous ex- 

 tent and depth ; conical mountains produced by eruptions ; 

 currents of lava which have flowed in every direction ; beds of 

 black and white rapilli, and tufa ; with the sulphureous fumes 

 from the Peak indicating an active state of combustion, which 

 in a moment may be the cause of new disasters. 



The lavas are of endless variety, and their appearance is 

 variously modified, according to the heat and pressure they 

 have been subjected to; and frequently there is a heaving-up of 

 the strata from central points, from which they dip away in 

 various directions as if they were elevated from the bottom of 

 the sea by the pressure of elastic vapours, which have changed 

 the lower strata of the island from their horizontal position to 

 their present one of great inclination, 



N.S. Vol. 8. No. 48. Dec. 1830. 3 K The 



