192 Scientific Proceedings, Royal Dublin Society. 



repetition of the shingle-layer ; practically all the rock is what I have 

 described as Tosca. it is well stratified, showing very even bedding ; in some 

 places, however, one can see layers cropping out up to several feet in thickness, 

 composed chiefly of marine shells. Where shingle occurs down the face of a 

 cliff it always seems to mark the remains of old river-beds. The pebbles 

 found are of a very different composition from those found on the pampa 

 the layers are limited in extent laterally, and rhey never run along the cliff 

 more than a few hundred yards in one place. There is, as a rule, distinct and 

 well-marked unconformity between this latter shingle and the surrounding 

 rock; and, when we consider that these tertiary river-beds are very lare and 

 only found in a few places, we realize that there is practically no resemblance 

 between them and the great layer of shingle found on the pampa. 



From these facts w-e can conclude that this layer of pampa shingle is a 

 unique formation, and that there is no parallel to be found in this country— 

 at least in the earlier rocks. Now, if this shingle-layer was a beach or marine 

 formation, as Darwin tried to prove, we should surely have found in the rocks 

 beneath some repetition of it formed when the country began to subside under 

 the sea or to emerge from it. Especially should this be the case when we 

 consider that these subsidences during tertiary time extended right to the 

 Cordillera, as is proved by similar marine formation being found well among 

 the mountains. Consequently, I think the marine theory must be abandoned. 

 The next point of importance with regard to this shingle-layer is that in all 

 the localities where I have examined it between the Santa Cruz river and the 

 Straits of Magellan 1 have found it unfossiliferous. 1 understand that to the 

 north of San Julian and Deseado in many places recent sea-shells are found 

 on the surface of this layer ; but from what I have gathered from intelligent 

 people who have visited the locality in question, these shells are not found to 

 any depth in the shingle. This would indicate that they represent a recent 

 subsidence long after the shingle had been spread out over the country, I 

 have myself noticed on many occasions mussel shells lying in heaps, mixed 

 with the excreta of sea birds, on the pampa upwards of two miles or more 

 from the beach, so that we should always bear in mind the possibility of 

 carriage by birds when considering isolated instances. 



II. 



Rdcdions hetwccn the shingle and the surface on which it was laid doivn. 



If we go round the point at the mouth of the Gallegos river and turn to 

 the north along the Atlantic coast, when we get as far as the first pampa 

 level we see, as we have said, all along the top a darkish-brown layer, lying 



