FOSSIL MAMMALIA. J 1 



mud which might have accumulated in channels, and on the surface of the 

 gravel, if this part of the plain had formerly existed as a harbour, such as 

 Port St. Julian is at the present day. The Guanaco, the only large animal 

 now inhabiting the wild plains of Patagonia, often wanders over the extensive 

 fiats, which are left dry at the head of the harbour during ebb tide : we may 

 imagine that the fossil animal, whilst in a like manner crossing the ancient bay, 

 fell into one of the muddy creeks, and was there buried. 



I have stated that existing species of shells are scattered over the surface of 

 this plain ; namely, Mytilus Magellanicus ; a second and undescribed species, 

 now living on the beach ; M. edulis ; Patella deaurata ; and on another part of the 

 coast, but having similar geological relations, Fusus Magellanicus ; Voluta ancilla; 

 and a Balanus .-—all these shells are among the commonest now living on this 

 coast. Although they must have been lying exposed to the atmospheric changes 

 for a very long period, they still partially retain their different colours. From 

 these facts we know, with certainty, that the superficial deposit, containing the 

 remains of the quadruped, has been elevated above the sea, within the recent 

 period. From the structure of the step-like plains, which front the coast, it is 

 certain that each step must have been modelled, subsequently to the elevation of 

 the one standing above it ; and, as the same recent shells occur on two higher 

 plains, we may, with safety, conclude, that the earthy matter, forming the sur- 

 face of this lower one, together with its embedded skeleton, was deposited long 

 after the existence of the present species, still inhabitants of the sea. Accord- 

 ing, therefore, to the chronology, taken from the duration of species among the 

 molluscs, the fossil quadruped of Port St. Julian must have been coeval, or 

 nearly so, with those from Bahia Blanca. 



Having now briefly described the principal circumstances in the geology of 

 the three districts, to which I at first alluded, I will conclude, by observing, that 

 the fossil mammalia of La Plata, Bahia Blanca, and Port St. Julian, must all have 

 lived during a very modern period in the geological history of the world. It is not 

 the proper place in this work to enter on any speculations, concerning the cause of 

 the extinction of so many gigantic animals. I will only here add, that there is the 

 strongest evidence against admitting the theory of a period of overwhelming 

 violence, by which the inhabitants of the land could have been swept away, and 



