266 DR. -A. GILLIGAX ON THE PETROGRAPHY OF [vol. lxXV, 



Geniculate or cordate twins are extremely rare, and only a few 

 cases of polysynthetic twins have been noted. 



Colourless rutile after leucoxene has been noted in rnany cases. 



Zircon is present in every bed that has been examined, but 

 is by far the most abundant in the micaceous and arenaceous 

 shales, fine-grained grits, sandstones, and flags. Of the true grit- 

 beds it is of most frequent occurrence in the beds comprising the 

 Middle Grits, notably in the grit of Addingham Edge and the 

 associated beds. While well- developed crystals are found in all 

 the beds, the highest proportion is in the micaceous beds, and here 

 the sharpness of outline is most striking. It would suggest, as 

 mentioned above, that they have been transported to their present 

 position enclosed in some mineral, possibly a ferriferous mica which 

 has decomposed and released the zircon. There is confirmation of 

 this hypothesis in the fact that, in several sections of the grits, 

 biotite is found enclosing zircon. The crystals vary much in 

 habit, the ratios of length to breadth varying from 2:1 to 6 : 1, 

 the greatest proportion being about midway between these limits. 



Lengths measured on good crystals : — "09, *18, # 25, -30, -35 mm. 



Forms observed. 



Prisms 100 with pyramids 111. 

 Prisms 100 and 110 with pyramids 311 and 111. 

 Prisms 100 and 110 with pyramids 111. 

 Prisms 100 with pyramids 111 and basal plane 001. 

 These last are somewhat rare. 



The terminations in many cases are rounded, owing to the 

 development of a large number of pyramid faces. 



No capped crystals have been found, although a A*ery careful 

 look-out has been kept for them 



One twinned crystal similar to that figured b}' Mr. R. H. 

 Ilastall & Mr. W. H. Wilcockson from the microgranite of 

 Threlkeld Quarry, near Keswick, 1 has been found. 



Zonary structure is common, though it does not seem to be 

 characteristic of any particular habit. The zoning is more 

 common in the central part of the crystals than in the outer 

 portions. 



Inclusions are numerous, and consist of the following types : — 



(1) gas and glass inclusions of various forms, a tubular form being common ; 



(2) other minerals, usually with regular outline, and including zircon (xeno- 

 time) and possibly apatite ; (3) negative crystals. 



It has very frequently been observed, when using a high magni- 

 fication, that the inclusions themselves contain inclusions, especially 

 those of tubular form. The inclusions are, in some cases, zonally 

 arranged, and lie with their long axes parallel to the faces of the 

 crystal. It is impossible to photograph these inclusions so as to 

 show their abundance in any single case, as they, of course, do 



1 Q. J. G. S. vol. lxxi (1915-17) p. 612. 



