350 R. M. Brydone — The Belemnitella mucronata Zone. 



Being interested as to the chemical nature of the unaltered 

 andesite, Mr. R. K. S. Mitchell, one of the senior workers in this 

 lahoratory, kindly undertook to carry out for me a determination of 

 the silica and bases present, which he did under the supervision of 

 an exceedingly skilled analyst, my friend Dr. Sidney A. Kay. 

 Mr. Mitchell's figures are as follows: — 



SiOa 



AlaOs 



FeaOs 



MnO 



CaO 



MgO 



NazO 



K2O 



61-37 



18-80 



5-46 



5-62 



1-88 

 3-83 

 2-05 



99-01 



Comparison of these results with the analyses given by Sir Jethro 

 Teall in his valuable papers on the Cheviot andesites (Geological 

 Magazine, 1883) will at once show in what striking fashion there is 

 chemical similarity between the Old Red " pitchstone porphyrites " 

 of the borders and the example described above from East Fife. 



In conclusion, it is my duty to express very cordial thanks to 

 Professor James "Walker, F.R.S., for having added to the equipment 

 of this Department several items of optical apparatus which have 

 enabled me to carry out the foregoing mineral determinations. 



III. — The Thiceness of the Zone of Belemnitella mucronata 

 IN THE Isle of Wight. 



By E. M. Brydone, F.G.S. 



N 1908 Dr. Rowe published an account of the Chalk of the Isle of 

 AVight' with a zonal map in which the zone of Belemnitella 

 mucronata was shown as entirely absent at some points but generally 

 present in substantial thickness. At the two ends of the island 

 actual measurements were made, of 150 feet at Culver Down. and 

 475 feet at the ISTeedles. How much of the latter thickness was 

 measured in the cliffs and how much is made up of somewhat less 

 satisfactory measurements at low tide on weed-covered reefs is not 

 clear, but at any rate there must be well over 300 feet in the cliffs. 

 This figure is sufficient to show how great a ravining of the 

 pre-Tertiary surface of the chalk would be required for the repeated 

 disappearances of the zone as mapped. It seems justifiable to pay 

 some critical attention to the evidence on which such a state of 

 things is alleged. 



It will be found on reference to the map in question that there 

 are five points at which the zone of Belemnitella mucronata is repre- 

 sented as disappearing completely, while in between them it swells 

 out each time to a very substantial thickness. (I do not include the 



^ "The Zones of the "White Chalk of the English Coast. 

 Wight " : Proc. Geol. Assoc, vol. xx, pt. iv, p. 209. 



V; The Isle of 



