XCU PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



the eighteen species of cephalopodous moUuscs, seventeen occur in 

 the French bed to which the comparison is extended, ten in the 

 Belgian, and thirteen in the German, there can be httle doubt of 

 the identity of the several beds ; and as regards the position of those 

 beds, I may observe, that, vv^hilst eight species descend to the upper 

 lias clay, or to a bed below the disputed sands, not one species rises 

 to the inferior oolite, as recognized by Wright and Oppel either in 

 England or on the Continent, though the said inferior oolite con- 

 tains no less than fourteen species of Cephalopoda in England, 

 twelve of which are also found in the corresponding bed in France, 

 though nearly absent from the beds of Belgium and Germany. If 

 thus the upper Cephalopoda-bed be attached to the oolites, the 

 demarcation-line must be carried down so far as to include even the 

 upper lias clay in the bed below the doubtful sands. Judging, 

 then, from the present evidence, I cannot but think there is much 

 reason for attaching this section of the strata in question to the 

 liasic series. 



Having referred to the authority of Dr. Albert Oppel in my 

 remarks on the paper of Dr. Wright, I will now briefly notice the 

 very important work of that author on the Jura-Formation. Geo- 

 logy has now arrived at a point where the " correlation of the 

 minuter divisions of the deposits of different countries necessarily 

 becomes the object of geologists." On the surface of the earth, as 

 it now is, we know that the deposit we see forming at one place can 

 afford us no positive information as to that which simultaneously may 

 be forming at another and distant place : each requires to be stu- 

 died by itself; and, were it not that we know as a fact the contem- 

 poraneity of their formation, we should often find great difficulty . 

 in proving it. The same comparison must be made between the 

 separate deposits of former epochs, though the difficulty is so much 

 the greater, as the positive knowledge which we possess as to the 

 time of deposition of recent strata is here totally wanting, and is 

 the thing to be proved. The term * correlation,' and the process it 

 expresses, have both been rendered familiar to us by the continued 

 labours of Mr. Prestwich on the Tertiaries, and others have fol- 

 lowed in the same line of inquiry, such as Murchison and Bar- 

 rande on the Silurians, the Sandbergers on the Devonians, and 

 Dr. Oppel on the Jura Formation. In this work he divides this 

 great and peculiarly interesting formation into three great divisions : 

 — 1. the lower jura, or lias; 2. the middle jura; 3. the upper 

 jura. He then proceeds to the consideration of the lower jura or 

 lias, and subdivides it also into three sections, namely, — 



Germany. France. England. 



1. Unterer Lias. Sinemurien. I^ower Lias. 



2. Mittlerer Lias. Liasien. Marlstone, or Middle Lias. 



3. Oberer Lias. Toarcien. Upper Lias. 



The Middle Jura is also divided into three stages or sections 

 thus : — 



