532 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



gation vein Dear the middle of the Reservoir Quarry. It 

 shows the relations of the medium and fine aplites, and the 

 fine and coarse grained pegmatites, the aplites as usual 

 lying in the middle of the vein in wavy streaks. Fig. 4 is 

 a sketch of the large irregularly oval segregation mass in 

 the upper great vein in the Reservoir Quarry, already 

 referred to, which is located near letter H in Plate XXXV. 

 It is an arched lenticular swelling of the vein, both the 

 pegmatite and aplite becoming thicker. Two veins con- 

 sisting of varieties of aplite branch off from the middle 

 downwards. The following is a measured section across 

 the thickest part of the vein at this point : — 



Pegmatite grading imperceptibly into 



normal essexite ... ... ... 12 inches 



Pegmatite with veins of aplite ... 24 „ 



Aplite medium to coarse 26 ,, 



Pegmatite 24 ,, 



Pegmatite grading into normal essexite 24 ,, 



Total 110 „ 



It has been possible to take a few photographs of the 

 segregation veins, and two of them are reproduced in the 

 plates to be found at the end of this paper. In Plate 

 XXXVII we see a portion of the upper great aplitic vein 

 takeD near the middle of the Reservoir Quarry. On either 

 side lie coarse and fine pegmatite and in the middle medium 

 and fine aplite. A little study of the photograph shows 

 that the arrangement of the different varieties of rock, 

 though somewhat irregular, is not devoid or order. The 

 medium aplite occurs only as a strip of varying width in 

 the middle of the vein; it is bordered by fine aplite, and 

 the pegmatite occupies the rest of the vein above and 

 below. The coarse aplite here occurs mostly in the shape 

 of bulging pockets. The dimensions of the vein measured 



