TRANSACTIONS OF SEOTION C. 701 



a great diversity of igneous rocks, varying from highly basic to acid types. Contact 

 phenomena of complex nature occur at the junctions of « and b. 



4. Bed Granite and Dyke Rocks, whose parallelism and extent of distribution 

 present one of the most conspicuous features of the Eastern Desert of Egypt, mark 

 the final eruptive action before Carboniferous times. 



5. Three periods of volcanic activity have been subsequently noted. 



(a) In Western Sinai in late Carboniferous times. 



(h) An undated series of eruptions interbedded with the base of the Nubian 



Sandstone or intrusive into it with marked contact alterations, 

 (c) The Basic Intrusions near Cairo and the f ayum, &c., which ai'e intimately 



associated with the Oligocene Continental Period in Egypt. 



6. The Chemical Composition and Optical Characters of Dolomite 

 Crystals from Algeria. By A. Hutchinson, M.A., Ph.D. 



Fine crystals of dolomite are found at Biskra. As the material is perfectly 

 colourless and transparent, it was thought that a careful chemic.il and optical 

 examination would be of interest. 



The results of an analysis were as follows : — 



or FeCO, . . . 1-22 

 „ CaCO, . . . 5409 

 „ MgCd, . . . 45-20 



100-51 



100-31 



The indices of refraction were determined by means of a prism cut from the 

 crystal which afforded material for analysis. The values obtained for the mono- 

 chromatic flames given by lithium, sodium, and thallium respectively were — 



Li Na Tl 



Ord. 1-5002 1-5022 1-5042 



Extraord. 1-6767 1-6813 1-6855 



7. A Protractor for constructing Stereographic and Gnomonic Projections 

 of the Sphere. By A. Hutchinson, M.A., Ph.D. 



The protractor, which has been designed for the use of students of crystal- 

 lographjr, consists of a rectangular strip of boxwood of any convenient length 

 and thickness, and of a width exactly equal to the radius of the sphere to be 

 projected. Experience has shown that 2-5 inches is a suitable radius for this 

 sphere ; the protractor must therefore be 2-5 inches wide, and should be about 

 12 inches long. A zero lino OD is drawn across it at right angles to its length, 

 at a distance from one end rather greater than the width of the protractor. 

 Taking as centre the point D, the opposite edge is graduated by means of a 

 circular dividing engine. The portion of the scale to the right of is divided 

 to degrees, and the divisions numbered as shown in the figure, in which, however, 

 for the sake of clearness, the finer divisions have been omitted. The divisions 

 from to A are exactly the same as those from to C, but are numbered 

 differently, each division between O and representing 2° instead of 1°. 



From the method of construction it appears at once that the distance from 

 of any division lying to its right is numerically equal to the tangent of the 

 corresponding angle (read on the upper row of numbers) when measured with a 

 scale for which the radius of the primitive circle DABC is taken as unit. Such 

 a scale is provided on the back of the protractor. Similarly, the distance from O 

 of any division on the scale OA is numerically equal to the tangent of half the 



