Vol. 59.] THE GEOLOGY OF PATAGONIA. 163 



Entrance Point, being apparently the inland continuation of the 

 lower 150 feet only. It consists of dull green sandstone with some 

 argillaceous matter, and contains several layers of calcareous con- 

 cretions arranged along the bedding-planes, which are only seen to 

 be dipping slightly southward by following the beds along the coast 

 of the Atlantic. The sandstone is highly felspathic, both orthoclase 

 and plagioclase being abundant. The grains are distinctly worn, 

 and comprise, in addition to the felspars, quartz, hypersthene, a 

 monoclinic pyroxene, and a small amount of zircon. The presence 

 of hypersthene, which forms the bulk of the heavier minerals and is 

 quite fresh, together with the plagioclase and monoclinic pyroxene, 

 is remarkable, since it suggests that the Patagonian Beds are partly 

 derived from contemporaneous andesitic tuffs or older tuffs and lavas. 



The top of the Patagonian Beds is excellently exposed in the cliff- 

 section, on a part of the coast at Monte Leon, well known locally as 

 the haunt of numbers of sea-lions. Fig. 2 (p. 162) is a section 

 showing not only the Patagonian Beds, but also a thick series 

 of estuarine deposits succeeded by the well-known Santa Cruz 

 mammaliferous deposits. 



The shell-bed, which does not extend for more than a quarter of 

 a mile, contains abundant invertebrate remains, some of which, 

 notably Turritella, are found in pockets in the overlying gypseous 

 mudstone, where they seem to have struggled for some time against 

 the unfavourable conditions. 



The estuarine series consists of yellow and white mudstones, 

 and false-bedded sandstones and grits. Plant-remains are abundant, 

 but only Fagus could be determined ; at the top of the series a few 

 specimens of Ostrea were seen projecting from the mudstone-bands. 



The white mudstone-bands of the Santa Cruz Beds 

 consist to a large extent of isotropic pumiceous l material; fragments 

 of felsite, 2 hypersthene, and monoclinic pyroxene also occur. 



III. The Igneous Eocks. 



Passing to the igneous rocks, we find that very little has been noted 

 as to their nature, and, with the exception of the basalt-plateaux, 

 not much is known of their distribution. I was able, during my 

 journey, to collect specimens from the moraines bordering Lake 

 Buenos Aires, and also to examine certain masses of intrusive rock 

 in the territory of Chubut. But I was unfortunate in not seeing the 

 mass at Port Desire (Puerto Deseado) described in detail by Darwin * 

 as a claystone-porphyry of metamorphic origin. 



At Port St. Helena a quartz -porphyry dyke occurs, which was 

 mentioned by Darwin 4 as closely resembling the Port-Desire rock, 

 and also the basal formations of the Chilian Cordilleras. 5 It trends 

 east and west, is 7 miles long, and three-fifths of a mile broad. 

 Although intruded into soft gypseous mudstones, this dyke does not 



1 Pumiceous tuff was noted by Ehrenberg in sediments farther north. See 

 Darwin's * Geol. Obs.* 2nd ed. (1876) p. 374. 



2 The word felsite is here used to denote fragments of the groundmass of 

 lavas and intrusive rocks in which the minerals are too small for determination. 



3 ' Geol. Obs.' 2nd ed. (1876) p. 436. 4 Ibid. p. 435. 5 Ibid, pp. 435, 473. 



