46 



mi. A. HOLMES 0^ THE PRE-CAMB1UAX [vol. lxxiv, 



with a lime- soda felspar would yield oligoclase, epidote, and more 

 free quartz, while any further excess of lime interacting with 

 ilmenite and the silica already provided, would readily produce 

 sphene. 



IV. The Crystallise Limestones. 



On the northern side of the Monapo Valley, below the three 

 peaks of Mount Tibwi (see fig. 6, below), Mr. R. L. Reid discovered 

 a large mass of crystalline limestone surrounded by hornblende- 

 Fig. 6. — Sketch-map of the Monapo District. 



gneiss that passes rapidly into the usual biotite-gneiss away from the 

 contacts. The boundaries of the limestone are by no means well 

 defined, for lenticular masses which are partly or completely 

 separated from the main body appear among the surrounding 

 gneisses, these having penetrated the limestone in a very irregular 

 manner. Moreover, both gneisses and limestone are cut by small 

 sill-like intrusions of a later microcline-granite. 



The crystalline limestone (No. 72, from the interior of the mass) is a grey 

 lustrous rock made up very largely of calcite. In thin section a few rounded 





