chap. xv. Section up the Valley, 547 



bling some of the highly metamorphosed beds at Ar- 

 queros : this bed was capped by, and appeared to pass 

 in its upper part into, rocks similarly coloured, con- 

 taining calcareous matter, and abounding with minute 

 crystals, mostly elongated and glassy, of reddish albite. 

 Seventhly, a conformable stratum of fine reddish por- 

 phyry with large crystals of (albitic ?) feldspar ; pro- 

 bably a submarine lava. Eighthly, another conformable 

 bed of green porphyry, with specks of green earth and 

 cream-coloured crystals of feldspar. I believe that 

 there are other superincumbent crystalline strata and 

 submarine lavas, but I had not time to examine them. 



The upper beds in this section probably correspond 

 with parts of the great gypseous formation ; and the 

 lower beds of red sandstone conglomerate and fossili- 

 ferous limestone no doubt are the equivalents of the 

 Hippurite stratum, seen in descending from Arqueros 

 to Pluclaro, which there lies conformably upon the 

 porphyritic conglomerate formation. The fossils found 

 in the third bed, consist of — 



Pecten Dufreynoyi, d'Orbig. £ Voyage, Part. Pal.' 



This species, which occurs here in vast numbers, according to 

 M. d'Orbigny, resembles certain cretaceous forms. 



Ostrea hemispherica, d'Orbig. £ Voyage, &c.' 

 Also resembles, according to the same author, cretaceous forms. 



Terebratula senigma, d'Orbig. ' Voyage, &c.' (PI. 

 XXII. figs. 10-12), and PL V. figs. 10, 11, 12 of this 

 work. 



Is allied, according to M. d'Orbigny, to T. concinna from the 

 Forest Marble. A series of this species, collected in several localities 

 hereafter to be referred to, has been laid before Prof. E. Forbes ; and 

 he informs me that many of the specimens are almost undistinguish- 

 able from our oolitic T. tetraedra, and that the varieties amongst them 

 are such as are found in that variable species. Generally speaking, 

 the American specimens of T. aenigma may be distinguished from the 



