THE TRAP DYKES OF THE ORKNEYS. 



899 



evolved on treatment with cold dilute HC1, but on warming, the evolution of gas was 

 renewed and rapid. To what extent the abundant secondary products have resulted 

 from the decomposition of the dyke rock itself, or have infiltrated from the surrounding 

 highly calcareous flagstones, it is impossible accurately to determine, but, from the 

 investigations of Merrill (XXII. , p. 214), we may assume that the effect has been to 

 increase the apparent percentage of alumina and to diminish that of silica and the 

 alkalies. This would make the accordance still more complete. 



An examination of anatyses 1, 2, and 3 shows how closely connected are these three 

 rocks which we have selected as types of different groups, and leads to the assumption 

 that they must have proceeded from the same igneous magma. In the monchiquite we 

 have a relatively high percentage of silica, and the alkalies, while lime, magnesia, 

 alumina, and iron, are slightly low. The others have the extremely low silica percentage 

 characteristic of the melilite rocks. In the alnoite potash predominates, in the melilite 

 monchiquite soda. The alnoite is richer in magnesia, lime, and iron oxides, the melilite 

 monchiquite in alumina. As decomposition has followed a similar course in these three 

 rocks, more reliance can be placed on a comparison between them, and their chemical 

 composition, taking into consideration the analyses of their constituents (see Table below), 

 may reasonably be expected to throw some light on their diversity of character. 



Chemical Analyses of Constituents of Monchiquites and Alnoites. 





1. 



2. 



3. 



4. 



5. 



Si0 2 



53-43 



38-56 



44-55 



36-42 



44-76 



Ti0 2 







2-85 



3-99 





A1 2 3 



20-86 





7-86 



17-92 



7-90 



Fe 2 3 



2-61 



1-36 



3-81 



283 



5-16 



FeO 





12-65 



4-53 



7-04 



1-39 



MnO 





0-11 



0-38 







MgO 



0-29 



44-37 



12-71 



20-52 



8-60 



CaO 



W4 





20-84 





27-47 



Na 2 



11-63 





1-29 



2-60 



2-65 



K 2 



2-51 





0-49 



6-54 



0-33 



H 2 



7-06 



2-91 





2-50 



1-42 



C0 2 



... 











99-53 



99-96 



99-31 



100-36 



99-68 



1. Glassy base of monchiquite Sta. Cruz Bahn (Hunter and Rosenbusch, XVI., p. 454). 



2. Olivine from alnoite, Ste. Anne de Bellevue, Montreal, Canada (anal. Prof. Harrington (Adams, 

 XVIIL, p. 273).) 



3. Augite from monchiquite, Rio de Ouro, Sierra de Tingua, Rio Janeiro (Hunter and Rosenbusch, 

 XVI., p. 462). 



4. Biotite from monchiquite, Norherig, Oberbergen (cited from Rosenbusch, XIX., p. 234). 



5. Melilite from melilite basalt, Hochbohl. (cited from Rosenbusch, XIX., p. 360). 



