390 Prof. Sedgwick on the [May, 



the fossils contained in the iron sand and the other beds imme- 

 diattely beneath the chalk. Secondly, to ofter some remarks on 

 the formations above the chalk. The observations on which the 

 remarks are principally founded were made during two visits to 

 the Isle of Wight ; one in the early part of the year 1819 ; the 

 Other in the early part of last summer. 



No paper in the Annals oj' Fhihsophj/ will be quoted, except 

 the one before alluded to ; it will, therefore, be only necessajy 

 to refer to the page in which any particular observation is con- 

 tained. I have the honour to be, Sir, 



Your most obedient servant, 



A. SeDGW ICK. 



I. Observations on the Formations between the Portland Oolite 

 and the Chalk. 



Since the publication of Mr. Webster's letters to Sir H. Engle- 

 field, the deposits between the Portland oolite and the chalk 

 have generally been subdivided into three formations. (1.) Iron 

 sand, a formation of ferruginous sand and sandstone. (2.; An 

 argillaceous deposit {Tetsivorth clay of Mr. Greenough). 

 (3.) Green sandy a siliceous deposit, containing subordinate beds 

 of limestone, &:c. ; generally characterized by the presence of a 

 certain quantity of green earth. This classification is indepen- 

 dent of all theory, and is only to be considered as a convenient 

 mode of arranging a great many similar beds, the geological 

 relations of which have been made out by actual observation. 

 The three formations may together be considered as the repre- 

 setitatives of the quader-sandstein of Werner, as they appear to 

 be associated with rocks of the same age, and agree with it 

 in some of their more minute characters. Many details must, 

 however, be supplied before even this point can be completely 

 established. From the whole analogy of the English coast, as 

 well as from the best accounts with which we are acquainted, 

 there can be no doubt but that some of the beds here described 

 are widely extended in the north of France. In many parts of 

 England, they preserve an extraordinary unity in their general 

 relations and external character. On the whole, if we follow 

 the great chalk escarpment towards the north-east, they gra- 

 dually thin off, and sometimes, perhaps, entirely disappear. A 

 few miles to the north of Flambro' Head, the chalk is seen in the 

 cliff reposing on the clay of the vale of Pickering, without the 

 intervention of any beds of sand or sandstone. With whatever 

 certainty the great relations of the three formations above-men- 

 tioned may have been estabhshed, errors may arise in fixing 

 points of detail from some of the following causes. (1.) The 

 colouring principles of the upper and the lower sand are some- 

 times wanting. (2.) The green particles which characterize the 

 Upper sand sometimes abound so much in the lower as to alter 



