Correspondence — A. H. BlooTufield. 



45 



by taking log sines for co-ordinates, the result being a series of parallel 

 straight lines. — Dr. A. Hutchinson and W. Campbell Smith: Labra- 

 dorite from St. John's Point, Co. Down. The large fresh crystals of 

 felspar, which occur in a basaltic dyke, have physical characters — 

 specific gravity 2'706, extinction on 010 and Ool — 23° and — 11° 

 respectively, refractive indices a 1-5630, /3 1-5665, 7 1-5712 — which, 

 agree closely with the position of the felspar in the plagioclase series 

 given by its chemical composition, which is approximately represented 

 by the formula 33 Ab 5 Or 62 An.— Dr. G. F. H. Smith: Apparatus 

 for preparing Thin Sections of Rocks. A description was given of the 

 apparatus recently made for the Mineral Department of the British 

 Museum. — Russell F. Gwinnell : Calcite Crystals from a Water Tank. 

 The crystals, which were deposited during the dry summer of 1911 

 from water derived from a spring in the marlstone of Belton Park, 

 near Grantham, Lincolnshire, averaged Q-lmra. in greatest diameter, 

 and showed the unusual unit rhombohedron form 1011. 



COE-K.EISiE'OIN'IDElSrCE. 



BEMBRIDGE LIMESTONE AT CREECHBARROW HILL.^ 

 Sir, — As Mr. Keeping's latest Report on Creechbarrow Hill leaves 

 the question of its true geological structure still somewhat uncertain, 

 I venture to draw attention to one or two points which may, perhaps, 

 be of some help towards arriving at a correct conclusion. 



1. If the Pipeclay is to be relied upon as a datura-line, some 

 idea can be formed of the approximate thickness of strata between it 

 and the Limestone above. 



2. Latest borings on the eastern flank have proved Pipeclay as 

 high up as 393 feet O.D., at a horizontal distance below the summit 

 of not more than 610 feet. Shown thus — 



'Summit,637. ft 

 0.0/ -^■- 



Limeslone 



Hon^onZal line 6lOft. 



Pipeclay juw-ed- 

 in these boreholes'. 



Diagram of Creechbarrow Hill, showing excavations and boreholes. 



1, 2, 3, 4. Trenches which showed stratified deposits of gravel, large flints, 

 sand, and clay, apparently dipping into the hill. 



N.B. Most probably the outcrop of the Pipeclay is much higher up, leaving 

 so much less space for later deposits. 



^ See British Association Report, Section C (Geology), Dundee Meeting, 

 September, 1912, on Mr. Keeping's further examination of Creechbarrow Hill, 

 Isle of Purbeck. (Geol. Mag., November, 1912, pp. 509-10.) 



