On the Metamorphic Rocks of Counties Sligo and Leitrim. 369 



On the other hand, the tremolite with crossed nicols exhibited 

 a deep blue tint, shading off into purple, and finally into golden 

 yellow, according to the varying thickness of the section. With 

 parallel nicols the colours were sulphur yellow, passing into light 

 blue ; the fibrous structure is very distinct. 



No. 2. Rock formed of alternating dark-green bands of serpen- 

 tine, and nearly black material. 



Microscope. — The appearance is very peculiar. The field shows 

 layers of light greenish-yellow serpentine, with net-like structure, 

 similar to that in the former specimen, but alternating with 

 black bands of magnetite bounded by jagged edges (Plate 

 XXXIII., Fig. 3). These bands are not continuous, but broken, 

 irregular, and variable in breadth, sometimes represented by mere 

 specks. There is also a large irregular mass of magnetite grains 

 enclosed in the serpentine in one part of the field (Plate XXXIII 

 Fig.l). _ 



Sometimes along the edges of the bands the magnetite seems 

 to be intimately associated with tremolite, which also occurs 

 in thin bands, being parallel to, or alternating with, the long 

 fibrous prisms. This is shown with a high power (one-fifth 

 objective), where, in one instance, the replacement of the tremo- 

 lite fibres by little black rods of magnetite dust is very clearly 

 shown. The proportion of magnetite in this specimen is unusu- 

 ally large. 



Polariscope. — With polarized light this section presents a very 

 beautiful appearance, owing to the surface being bespangled with 

 varying depths of blue, brought out with crossed nicols. The 

 efiects are similar to those described in the case of specimen 

 No. 1, and need not be repeated. 



No. 3. Dark-green dense serpentine rock, traversed by approx= 

 imately parallel bands of light-green silky tremolite (?) 



Microscope — In this instance the light yellowish -green serpen- 

 tine, and colourless tremolite with fibrous structure, are inti- 

 mately mixed throughout. The tremolite occurs in irregular 

 —nearly parallel — bands, and the hair-like prisms are arranged 

 perpendicular to the waUs of the inside bands; and, as the 

 section has been cut transversely to these bands, the hair-like 

 prisms cross the field from right to left. The serpentine has a 

 reticulated structure. 



