702 



TRANSACTIONS OF SECTION G. 



angle as read on the lower row of numbers, and the divisions themselves correspond 

 to degrees stereograph ically projected on a diameter. 



Since the graduation of Op is identical with that of the ordinary rectangular 

 protractor, this scale may be used in the usual way for setting-otF angles. So far 

 as special applications are concerned, the two scales Op and OA, used separately 

 cr in conjunction, enable us to solve with ease the following problems of con- 

 struction : — 



(1) To project on any diameter the position of a point P, of which the angular 

 distance from the primitive or from the North pole is known. In the figure P ia 

 the projection of a point 50° from the primitive. 



(2) To draw through any point P a great circle BFD perpendicular to that 

 projected in AOC. The centre of this circle lies at n, a point which has the same 

 reading on the right-hand scale that P has on the left. 



(3) To find the pole P' of any projected great circle BPD, or in general to 

 find the point 90° removed from a given point and lying in the same diameter. 



(4) To find the projected position p of a face pai'allel to a face of which the 

 projected position P is known. In other words, to find the point diametrically 

 opposite to any given point. 



(5) To draw small circles about any given point. 



The protractor may also be employed with advantase in constructing gnomonic 

 projections, as points of which the North polar distances are known can be plotted 

 at once from the scale Op. It will also be found useful in measuring the angle? 

 between faces and zones on the completed projection. 



8. Notes on the Microscopical Structure of the Derbyshire Limestones. 

 By H. H. Arnold-Bemrose, Sc.D., F.G.8. 



These notes form only a brief sketch of the results of a cursory examination 

 of some four hundred thin slices of Derbyshire limestone in the author's collection. 

 The specimens have been examined from a petrographical lather than from a 

 paleeontological point of view. The classification of microscopical types given in 

 this short paper must be considered as provisional because of the few specimens 

 examined from a large area of country, and from a thickness of over 1,800 feet 

 of limestone shales and mountain limestone, in addition to the Permian lime- 

 stones. 



I. Limestone Shales and Mountain Limestone. 



These may be divided into fine and coarse grained rocks. 



Fine-grained limestone, as eeen in the black lim^stoiie ghales in the black 



