168 MR. J. B. SCRIVENOR ON THE [May IQO3, 



patches and ' pools ' of pebbles which sometimes occur, that the 

 cap is very thin and not continuous. With the pebbles numbers of 

 Ostrece are found, smaller than those obtained at Port Madryn, and 

 probably derived from the underlying rnudstone. A poor natural 

 section occurs at Pozo del Sur, on the road to Camerones Bay, in the 

 bed of one of the small streams caused by the melting snow in 

 spring-time. Doubtless many more such sections exist, but since 

 the materials have been rearranged by the stream and mingled 

 with fragments of the rnudstone underneath, they are of little 

 interest. On the Chubut pampas the pebbles are chiefly quartz- 

 porphyries with a reddish matrix ; this type prevails for quite 

 18 leagues. On approaching Mr. Greenshield's estancia in Camerones 

 Bay a yellow quartz-porphyry is seen to take the place of the 

 red. It first appears in quantity in the Canadon Salado, and be- 

 comes more and more abundant as one approaches the estancia ; in 

 fact, south of Canadon Davis, between Caiiadon Salado and the 

 estancia all the larger pebbles, some as big as a man's fist, are of 

 this type. In some of the porphyry-pebbles quartz and felspar are 

 equally developed ; in others, one or other of these phenocrysts 

 predominates. At Camerones I noted, besides, pebbles of breccia, 

 banded tuffs, and occasionally of rhyolites. On the pampa at 

 Malespina, west of Camerones Bay, dark, spherulitic, and glassy 

 pebbles were noted in addition to the porphyries (probably acid 

 lavas), and also breccias containing portions of these lavas. In a 

 canadon hard by a large granophyre-pebble was found, and several 

 others of agate. In the valleys of the Chico de Chubut and 

 Senguerr the pebbles remain of the same type, but on the Rio Mayo 

 a change sets in — biotite-granite and gneissose rocks appearing. 



At Santa Cruz the river has piled up a bank of the pebbles by the 

 settlement ; among them foliated and contorted rocks, hornblende- 

 gneiss, rhyolites, and tuffs are common. At Monte Leon quartz- 

 porphyries are, as in every locality so far noted, the most conspicu- 

 ous element ; they vary in colour, being yellow, brown, or more 

 rarely red. One pebble of a sheared porphyry was collected ; of 

 the other pebbles the following are typical : — A holo crystalline rock 

 in which the quartz and orthoclase have crystallized simultaneously, 

 rich in epidote, but not revealing the nature of the original ferro- 

 magnesian mineral; a light-green pebble, apparently a highly- 

 altered and sheared sediment ; a beautiful spherulitic pebble, with 

 nuclei of quartz and one or two grains of orthoclase ; a large brown 

 pebble showing bands of felspathic material, disturbed by several 

 small dislocations, recalling the rock found in situ on the Eio Chico 

 de Chubut ; a green gneissose rock ; and a coarse-grained hornblende- 

 granite, with pink orthoclase. 



A good section of the Tehuelche Pebble-Bed is exposed in the 

 cliffs bordering the Rio Santa Cruz below the tide-limit on Don 

 Pedro Richmond's estancia, where it occasionally exceeds a thick- 

 ness of 10 feet. The height of the cliff above high- tide mark is 

 not more than 150 feet at the spot where I examined the section. 

 Here the calcareous cement mentioned by M. Mercerat is well 



