2-52 MR. J. .V. DOUGLAS OX OEOLOGICAL SECTIO>'S [vol. Ixxvii, 



horizons. The beds are best examined in the spur of Salto del 

 Fraile (the Friar's Leap) and in the chffs on the south side of the 

 Caleta de la Herradura, where they are again seen to have a 

 constant dip south-south-westwards corresponding to that observed 

 on San Lorenzo. 



Near the base of the succession occurs a series of coarsely-bedded 

 quartzites crowded with the casts of U-shaped annelid-tubes 

 i^GlossifiiRgites liahicJii hi^^on)?- These are frequently arranged 

 in well-defined layers, each successive colony doubtless represent- 

 ing a pause in the deposition of the bed, possibly due to seasonal 

 variation. In other cases, where sedimentation appears to have 

 been more continuous, individual tubes are seen to have undergone 

 a periodical displacement upwards, resulting in a repetition of the 

 U's in a vertical series. The whole formation is suggestive of 

 deposition in shallow water between tide-levels, such as might Avell 

 occur on a sandy beach, crowded with Avorm-casts, at the present 

 <iay. 



In addition to these annelid-tubes, other casts occur, the origin 

 of Avhicli is less certain. One type in particular, especially abun- 

 dant on the lower surface of a quartzite-band at its junction with 

 an underlying black shale, consisted of numerous freeh^-intersecting 

 tubes, about 1 inch in diameter and more or less circular in cross- 

 section, the interior of which was made up of countless ramifying 

 tubules of almost microscopic dimensions. 



The quartzites of Salto del Fraile are overlain by a succession of 

 black shales, mottled quartzites, and impure limestones forming 

 the Morro Solar, from which Prof. Lisson has obtained a rich 

 fauna including several species of ammonites, such as' Hoplites 

 (AcaRfliodiscHs) raimoiidii Gabb, H. (A.) pjluecl^eri Lisson, and 

 H. lorensis Lisson. 



The beds are pierced in many places by a fine-grained basic 

 porphyrite, occurring in the form both of vertical dj^kes and of 

 intrusive sills. Owing to the jointing of the igneous rock, how- 

 ever, these have offered less resistance to erosion than the quartzites, 

 and, where fully ex])osed to the action of the sea, their former 

 extension is often shown merely b}" open fissures. 



Kelying chiefly on the evidence afforded by the above-mentioned 

 ammonites and the fossil Hora of the Caleta del Paraiso, Lisson 

 and Steinmann regard these beds of Salto del Fraile and San 

 Lorenzo as being of Neocomian age ; and in support of this view it 

 may be noted that the sub-genus AcantJiodiscus makes its appear- 

 ance in Europe in Lower Valanginian times and Pictet's species 

 A. onalhosi, Avhich seems to bear the closest resemblance to 

 A. pfiueclceri Lisson, is one of the earliest-knoAvn forms. 



A study of the Trigonias, moreover, throws further light on this 

 question (see pala^ontological note). Of these, Trigonia lorentii 

 Dana is of chief importance, its interest lying in its similarity to 



^ See also H. Douville, ' Perforations d'Annelides ' Bull. Soc. G-eul. France, 

 .<5er. 4, vol. vii (1907) pp. 361-70 & pi. xii. 



