276 MESSRS. A. HARKEE AND J. E. MARK ON 



I. II. 



«iO^ 68-55 68-54 



Al,()., '... 16-21 15-&2 



^6^03 2-26 2-26 



FeO not estimated — 



MnO 0-45 0-52 



MgO 1-04 0-93 



CaO 2-40 1-99 



Na^O 4-08 4-23 



K^O 4-14 4-45 



H,0 not estimated not estimated 



99-13 98-74 



I. Shap-Fell granite, bulk-analysis from an average specimen 

 weighing 10 lb. ; anal. J. B. Cohen. 

 II. Calculated composition obtained from the analyses of the 

 porphyritic felspars and the groundmass given below (p. 278), 

 on the supposition of one part of porphyritic crystals to nine 

 of groundmass. 



The specific gravity of a specimen of the "light " Shap granite 

 was found to be 2*687. 



The microscope reveals several minerals in addition to those 

 enumerated above *. 



Apatite is present in all the slides examined. It occurs in little 

 prisms with hexagonal cross-section, and sometimes in very slender 

 needles. 



Zircon, in small quantity, is also a constant constituent, forming 

 small prisms terminated by an obtuse pyramid. These two minerals 

 are always the earliest products of consolidation, and contain no 

 inclusions. 



Magnetite is usually present, in little octahedra, in clusters of 

 partly-developed crystals, or in less regular patches. The bulk of 

 the mineral has separated at an early stage, but sometimes a portion 

 is seen to mould the mica and later minerals. 



Sphene is always present and often abundant. It commonly 

 shows good crystal forms ; namely, n (123), c (001), and y (101), in 

 Miller's notation ; but twinning is not met with. The cleavage- 

 traces are often apparent, making acute angles with the bounding 

 lines of the section. The colour in thin sections is light to 

 moderately deep brown, with well-marked pleochroism. Longitu- 

 dinal sections show the three forms mentioned, n being the best-deve- 

 loped : one of the axes of elasticity (a) is nearly parallel to the length 

 of such a section, and vibrations in this direction give a pale straw 

 colour, in the perpendicular direction a light reddish brown. Trans- 

 verso sections are parallelograms bounded by n-faces only, and show 

 a deeper colour than the others, with slightly less dichroism. The 



absorption formula is — ^ 



y > jj > a. 



* The specimens illustrating this paper, with about 150 sHdes, are in the 

 Woodwardian Museum at Cambridge. The figures given in square brackets 

 refer to the numbers of the slides. 



