VI 



THE GEOLOGIST. 



of subjacent beds. To use Mr. Murchison's own words — 

 "M. D'Orbigny points out the modifications of the species 

 through time and space, and shows the relation which exists 

 between certain forms and the beds which contain them. He 

 recognises three new creations or replacements of the species 

 of Ammonites during the cretaceous period ; and thus estab- 

 lishes on zoological data, these divisions of natural 

 groups ; the neocomien, the gault, and the upper greensand 

 and white chalk ; and he estimates that in this triple succes- 

 sion of deposits, the Ammonites gradually decrease according 

 to the numbers, seventy-five, forty-two, and twenty-seven, to 

 disappear finally with the uppermost chalk or Maestricht beds, 

 a?id before the tertiary epoch. 



The second point of interest to which the labours of geo- 

 logists have been directed, refers to the study of the older stra- 

 tified rocks, including the Silurian,^ Devonian, and Carboni- 

 ferous series. 



We need not recapitulate the general features of so called 

 " Silurian groups,^' which Mr. Murchison has long since 

 pointed out in his valuable work upon the subject; but during 

 the present year, geologists seem to have arrived at the con- 

 clusion, that the term Cambrian^' must cease to be used in 

 zoological classification^ it being synonymous in this sense ivith 



Lower Silurian^" subsequent researches having proved 



* We have recorded our opinions on the above term in the monthly no- 

 tice for May, 1842 ; (v. Geologist, p. 120), and although the reason urged 

 by Mr. Murchison for its recognition, seems plausible enough, "thatitislia- 

 ble to no misconceptions, as it is simply a geographical name, derived from 

 a region containing newly deposed types of succession," we still adhere 

 to the opinions expressed in the article alluded to, that a local term can- 

 not be applied for general comparison, and that in fact, it is a fallacy to 

 talk of the " Silurian rocks of Russia." We must, therefore, endea- 

 vour to detract from a British Geologist's boast, " in restoring to general 

 currency the word Silurian, as connected with great glory in the annals of 

 his country." 



