436 Fresh'jcater Formation at Copford, 



philosophical fact, that the atmosphere of Elysium and the 

 Fortunate Islands consists of the protoxide of nitrogen. 



But, independently of the chemical objections that we have 

 urged, there are many derived from the physical constitution 

 of bodies, such as the arrangement of the metallic nucleus ; the 

 impossibility of water ever penetrating to it ; the improbability 

 of the presence of atmospheric air, and the doubt whether 

 any chemical action could be developed under great pressures. 

 Each of these might, and the sum of them would, lead us to 

 the conclusion that the theory is doubtful, if not visionary. 



Another hypothesis attributes volcanic eruption to central 

 heat. The cause of this central heat is variously accounted 

 for by geologists, according to their particular notions or 

 fancies. Some attribute it to one cause, and some to another ; 

 but, whatever may be the origin of the singular phenomenon, 

 it has, we think, been proved that the interior heat of the 

 earth increases from its surface. It is veiy evident that this 

 fact would afford an ingenious theorist the opportunity of 

 creating a plausible hypothesis. We shall not attempt to 

 explain the almost endless variety of opinions that have been 

 entertained and defended upon the general idea that central 

 heat is the cause of volcanic activity, as this article has already 

 exceeded the space usually allotted to an individual subject; 

 and, in closing the communication, it cannot be denied that 

 the information already obtained, in relation to the activity of 

 volcanoes, is too vague and limited to warrant the formation 

 of a theory built upon observation. 

 July 10. 1834. 



Art. VIII. A Notice of some of the Contents of the Freshwater 

 Formation at Copfordy near Colchestery Essex. By J. Brown, 

 Esq. 



Mr. Wood has, in p. 275., invited some correspondent in 

 this neighbourhood to communicate notices on this subject. 

 I, for one, have pleasure in offering the results of my own 

 observations. 



From what attention I have given to this locality, I have 

 only been able to obtain perfect specimens of the Valvata 

 piscinalis. Some of these freshwater beds are mainly com- 

 posed of fragments of shells, but in such a state that it is 

 almost impossible to ascertain their species. Among the sand 

 and small fragments are found numerous opercula (probably 

 belonging to the genus Paludina), but no entire shell: the 



