PETROGRAPHICAL DESCRIPTION OF THE ROCK SPECIMENS. 



I.— ROCKS OF TENERIFE. 



Few of the volcanic islands of the Atlantic have been the object of such important 

 geological inquiries as Tenerife. It inspired L. von Buch's theory of elevation craters, 1 

 and since that time it has been very often visited by geologists. A large number of 

 scientific papers have been devoted to its description, among which one of the most 

 important is the remarkable monograph by von Fritsch and Reiss. 2 More recently 

 G. A. Sauer has given a detailed lithological description of the phonolites collected at 

 Tenerife by von Fritsch. 3 



The Challenger Naturalists, on a short visit to Pico de Teyde, 4 collected some 

 specimens, the description of which must, in the absence of stratigraphical details, be 

 limited to a few of those mineralogical and lithological features presented by some of 

 these rocks, that from a petrological point of view deserve to be made known. 



Near Puerto d'Orotava, basaltic scoria? are found; they are greyish-black, rough to the 

 touch, and vesicular ; the vesicles, lined with a siliceous coating, measure 

 from 2 to 3 mm. in diameter. None of the constituent minerals can be 

 detected with the naked eye. With the microscope crystals of augite 

 and rare crystals of olivine appear as elements of the first generation. 

 The rather large crystals of augite show very fine examples of polysyn- 

 thetic twinning, and were it not for the colour of the section, they might, 

 at first sight, be taken for felspathic lamellae twinned according to the 

 albite law. The adjoining figure (fig. 1) shows some of those elongated 

 sections of augite. The portion marked A is sensibly perpendicular to an 

 optical axis ; the portion B extinguishes in a direction parallel to the 

 lengthened edges. In the upper part of the figure the prismatic cleavage 

 is seen ; at other points irregular fractures are observed, like those seen 

 in sections of sanidine. Nearly all the sections of augite are twinned as 

 in the figure ; sometimes they are broken, and the fragments have been 

 displaced. Olivine is rather rare, and the outlines of its sections are faint ; neiirPue rt°<rorotava. 



*- Section of augite with 



the mineral is altered into serpentine, and the cracks are filled w ith pol ™ rnthetict ™ nin s- 

 calcite. A great deal of magnetite is found, but larger opaque patches ought to be 



1 L. von Bucb, Physikalische Besehreibung der Canarischen InselD, Gesaminelte Schriften, Bd. iii. p. 229, 

 Berlin 1877. 2 Von Fritsch und Reiss, Geologische Besehreibung der Insel Teneriffe, 1868. 



3 Sauer, Zeitschrift f, d. ges. Naturw., Bd. xlvii., Halle 1876. 



4 See Narrative of the Cruise of H.M.S. Challenger, vol. i. p. 53. 



(PHYS. CHEM. CHALL. EXP. PART VII. 1889.) 1 



