THE TRAP DYKES OF THE ORKNEYS. 



873 



green chlorite, mixed with calcite and limonite, is rather abundant, and fills up the inter- 

 spaces between the felspars of the groundmass. 



Specimens from nearer the margin contain felspar phenocrysts (partly of plagioclase) 

 in a groundmass of long lath-shaped felspars, for the most part plagioclase, though 

 simply twinned sections are frequent. The structure is trachytic and markedly fluidal 

 (PI. I. fig. l), the felspars of the groundmass having their long axes parallel and 

 winding in streams around the phenocrysts. Chlorite, with limonite and calcite, are 

 here more abundant, but the nature of the original component from which derived is 

 not determinable. 



At the extreme margin the rock is finer grained, long narrow crystals, both 

 orthoclase and plagioclase, most of them with forked irregular terminations lying in a 

 groundmass still in some measure glassy, but filled with minute pointed felspar 

 microliths arranged in no definite order. 



The difference between this rock and even the most felspathic of the camptonite 

 dykes is so marked as entirely to justify its recognition as the type of a distinct group. 

 The predominance of felspar and abundance of orthoclase, the paucity of ferro- 

 magnesian ingredients, the pale colour and low specific gravity (2*653), all support this 

 view. In some of the sections the resemblance to the bostonites is very close, but in 

 others the abundance of plagioclase and of chlorite show that the rock is of a more basic 

 character than any bostonite with which I have been able to compare it. Needless to 

 say, the sections show no quartz. The presence of glassy material in the selvage is also 

 a feature not characteristic of bostonite dykes. 





1. 



2. 



3. 



4. 



... ! 



1 

 1 



5. 



I 



1 



Si0 2 



52-00 



62-28 



63-25 



67-16 



56-50 



Ti0 2 



•98 









0-85 



A1 2 3 



18-06 



19-17 



22-12 



14-53 



18-14 



Fe 3 



2-18 



3-39 





4-17 



3-12 



FeO 



5-14 









2-86 



MnO 



•25 











MgO 



2-84 



tr. 





0-41 



1-22 



CaO 



4-59 



1-44 



0-56 



1-26 



3-38 



Na 2 



3-78 



5-37 



6-29 



5-55 



5-28 



K 2 



4-68 



5-93 



5-92 



6-10 



1-60 



H 2 



1-84 



233 



0-93 



1-10 



1-26 



PA 













C0 2 



3-59 









5-11 



99-93 



99-91 



99-07 



100-28 



99-32 



VOL. XXXIX. PART IV. (NO. 33). 



1. Onston Ness, Orkney. 



2. Lake Champlain, U.S.A. (IX. p. 20). 



3. M'Gill College, Montreal (cited from XIX. p. 211). 



4. Chateaugay Lake, Clinton Co., N.Y., U.S.A. (cited from XIX. p. 211). 



5. Maena, Gran, Christiania (VIII. p. 207). 



6 T 



