66 Dr A. Thomson on the 



equal length, and separated by great geographical distances, 

 and the melting of the snow and the rainy seasons being go- 

 verned by differences in their latitude and elevation, they are 

 consequently subject to overflow at different periods." 



Something of the same has been observed by other travel- 

 lers on the lower banks of the Amazon, where there is a 

 greater distance between the tributaries, and greater varie- 

 ties in the periods of flood of the various affluents, — a layer 

 of deep tenacious clay, alternating with various coloured sands 

 and gravels, being here a common occurrence. 



This may so far tend to explain appearances in the dilu- 

 vium of our own neighbourhood, around Edinburgh, where al- 

 ternations of clay, sand, and gravel, are by no means uncom- 

 mon. A good example of this we have at the clay deposit of 

 Portobello, especially in the section on the north or left hand 

 of the road, and which is now being wrought as a brick-work. 

 There may be seen a series of layers of silicious sand, of 

 about six inches in depth, alternating at regular intervals 

 with a depth of one to two feet of stiff tenacious clay. The 

 only fossil I have ever been able to detect in this clay was a 

 specimen which I now exhibit, and which appears to be a 

 cyclas. Three casts of the same species of shell were also 

 found, but no traces of the fragile shells remained. This 

 shell was found in the bed to the right of the road, and in a 

 solid mass of clay, about six feet from the surface. 



On the Discovery of a Frog in New Zealand. By Arthur 

 Saunders Thomson, M.D., Surgeon 58th Regiment. 

 Communicated by the Author for the Edinburgh New Phi- 

 losophical Journal. 



In the Fauna of New Zealand, compiled by J. E. Gray, 

 Esq., of the British Museum, and appended to Dieffenbach's 

 Travels in New Zealand, published in 1842, it is stated, on 

 the authority of Mr Polack, that " toads and frogs are not 

 uncommon, especially near the mountain districts, but he be- 

 lieves they do not differ from the species in Europe." With 

 this remark before his eyes Dieffenbach states, " they have 



