40 J. Stansfield—New Mode of Occurrence of Scapolite. 



Interpretation. — The explanation of the occurrence is simple. 

 The pegmatite, before consolidation, assimilated a small amount 

 oi the limestone into which it intruded, and the assimilated 

 lime gave rise to the formation of the lime-bearing silicate 

 scapolite, instead of orthoclase, as in the rest of the vein. The 

 chlorine required for the formation of the scapolite was an 

 original constituent of the pegmatite, and is seen to have been 

 capable of transmission, at least to a small distance, into the 

 country rock, as shown by the presence of scapolite in the 

 altered limestone of the ore mass. 



A partial analysis of the scapolite of the pegmatite vein, 

 which has a specific gravity of 2*690 fpycnometer], shows it to 

 contain 16'24 per cent of CaO. This, taken in conjunction 

 with the fact that the feldspar of the vein is orthoclase, clearly 

 indicates that its formation is a result of the assimilation of 

 lime from the country rock. 



This occurrence, in addition to the novel features described 

 above, throws an important light on the problem of assimila- 

 tion by igneous magmas. It shows that in the case of even a 

 small vein of pegmatite, such as is usually regarded as being 

 intruded at low temperatures, speaking relatively of magmatic 

 temperatures, the absorption of lime from a limestone country 

 rock can take place, and that also, which is the important 

 point, the absorbed lime may go toward the formation of lime- 

 silicates and not merely be re-deposited as calcite. If assimila- 

 tion may take place on the small scale represented in this 

 occurrence (involving only a few cubic feet of magma), it is 

 probable that on the larger batholithic scale it may play an 

 important part in the change of the average composition of a 

 large magma-reservoir, and so become an important factor in 

 the differentiation of rock types. Limestone is a rock which 

 lends itself to assimilation perhaps more readily than any 

 other. Some of the possibilities arising from the assimilation 

 of limestone have been discussed recently by Daly.* The 

 present paper can be regarded as a proof of the possibility of 

 assimilation and as showing that assimilation of limestone is a 

 comparatively easy process requiring temperatures that are 

 easily within experimental reach. So that the experimental 

 proof that new rock types of the more restricted kinds can be 

 formed from an average magma by the assimilation of lime- 

 stone should now be undertaken in the laboratory. 



McGill University, Montreal. 



*Bull. G. S. A., xxi, pp. 87-118, 1910. 



