498 G. E. Tilley — MetamorpJiism of 



involved is in accord with both field and microscopic evidence for 

 the majority of types investigated. Where, however, the rocks 

 are intimately penetrated by quartz strings, complete decarbonation 

 cannot necessarily be inferred, prior to the invasion by silica 

 solutions from the granites, and in view of the variability of com- 

 position of sediments of this type it is probable that the expulsion 

 of the last traces of carbonate minerals from some of these types was 

 accomplished by siliceous invasions. 



Referring to the equation (c), it will be seen that the diopside 

 is shown to contain the Tschermak molecule, and the validity of 

 this equation would be considerably strengthened if it were possible 

 to detect the presence of alumina in these diopsides, apart from the 

 fact that excess alumina may enter anorthite in many cases. The 

 optical characters shed little additional light on this question 

 beyond confirming that a pure diopside is not involved ^ (p. 493). 



In the case of detrital biotite being a source of some of the MgO 

 of the pyroxene, there may be no available alumina to be dealt 

 with, thus : — • 

 K^Mg^Al (SiOJa + 2 CaCOg + 7 SiOo = 2 CaMg (SiO,,)^ + 



2 KAlSigOe + 2 CO2. 

 But as in most biotites part of the K is replaced by H, this would 

 not be the general case. 



The remaining minerals accompanying the diopside-microcline 

 association call for some remark. 



Bytownite is characteristic. 



With the utilization of all magnesia from the dolomite molecule 

 residual calcite is free to react with alumina and silica, in the form 

 of sericite or kaolin, and anorthite may thus arise, e.g. : 



CaCOs + AI2O3 2 SiO.,, 2 H^O = CaAl^Si^O^ + 2 H,0 + CO^. 

 kaolin anorthite 



If sericite be the source of alumina, the potash would yield micro- 

 cline, and the soda of the sericite could be distributed between the 

 microcline and anorthite. (See also the equations already set down.) 



With the dwindling of plagioclase in these rocks, a member of 

 the scapolite group takes its place, and although these two minerals 

 may occur together the rocks are usually characterized by an 

 abundance of one or the other. The scapolite of these rocks has been 

 shown to approximate to meionite in composition. 



In view of the recent work of Borgstrom ^ and Sundius ^ on the 

 constitution of Finnish and other scapolites, in which they show 



^ Diopside microcline rocks have been described by J. S. Flett (" The Geology 

 of the country around Bodmin and St. Austell " : Mem. Geol. Surv., 1909, 

 p. 100), and by E. B. Bailey (" The Geology of Ben Nevis and Glen Cos " : 

 Mem. Geol. Surv., 1916, pp. 190-4), and the associations in those cases would 

 seem to demand the presence of an aluminous diopside as these investigators 

 have suggested. 



2 L. PI. Borgstrom, Zeitschr. Kryst. Min., vol.liv, 1914, p. 238. 



3 N. Sundius, Bull. Geol. Inst. Univ. Upsala, vol. xvi, 1919, pp. 96-106. 



