48 



DR. A. HOLMES ON THE PRE-CAMBEIAN [vol. lxxiv, 



These examples of contact-metaniorphism illustrate the important 

 principle that magnesia must be saturated with silica before lime. 

 Where silica is abundant diopside is formed ; but, when it is 

 deficient, then tremolite or actinolite is produced. Mr. E. B. Bailey 

 has recently exemplified the same principles in connexion with the 

 contact-metamorphism of the Ballachulish Limestone. 1 



The crystalline limestone (No. 221) of the Ampwihi Crossing 

 has already been mentioned in relation to its environment (p. 38, 

 and figs. 4 & 5, pp. 38-39). The rock is evenly granulose in 

 texture, and is composed of irregular grains of calcite averaging 

 5 mm. in diameter, with interstitial buff-coloured forsterite and a 

 few dark blue-grey crystals (2 mm. in axial length) of spinel. 



In thin section (PL VIII, fig. 5) the rock is seen to be made up chiefly of 

 calcite and forsterite, the latter mineral occurring in rounded masses and small 

 blebs that are serpentinized along cracks. Associated with the forsterite 

 there are often yellow granules of chondrodite, pleochroic from orange-yellow 

 to pale yellow or colourless. The maximum extinction-angle is about 30°, 

 thus distinguishing the mineral from other members of the humite group. 



Other minerals present are diopside, in colourless granules sometimes 

 showing cleavage, and having an extinction -angle of about 40° ; phlogopite, 

 in golden spangles scattered sparingly through the rock ; magnesia-spinel, 

 colourless in thin section, with a refractive index of 1*72; and finally 

 scapolite, in grains with elongated inclusions parallel to the length. This 

 mineral is optically anomalous, giving a biaxial figure of negative character 

 with a small angle between the optic axes, and slightly inclined extinction. 

 The refractive index is nearly 1*6 and the specific gravity 2*72, so that the 

 composition is about that of meionite. 



A partial analysis of the forsterite gave the result set forth in 

 column A of the following table. Other analyses of forsterite 

 from crystalline limestones occurring in Ceylon and elsewhere are 

 appended for comparison : — 



Table IV. 



A. Forsterite from Ampwihi Crossing, Mozambique. (An. A. Holmes.) 

 E. Forsterite from Hakgala, Ceylon. (An. G. T. Prior, Q. J. G. S. vol. lviii, 

 1902, p. 415.) 



C. Forsterite from Bolton, Mass. (U.S.A.).') 



(An. Brush.) (' Handbuch der Mineralogie ' 



D. Forsterite from Monte Somma. C C. Hintze, vol. ii (1897) pp. 5-6. 



(An. Micrisch.) ) 



1 ' The Geology of Ben Nevis & Glen Coe ' Mem. Geol. Surv. Scotland, 

 1916, p. 193. 



