312 



MKSSRS. A. HARKER AND J. E. MARR ON 



each crystal a division into several distinct " fields " with different 

 optical orientation. In addition to this there is a zonary banding 

 in polarized light, owing to the successive concentric shells of the 

 crystal differing in amount of double refraction. (See fig. 5.) 



Pig. 5. — [1211], Douhhj-refracting lime-garnets in the idocrase- 

 garnet-rocl\ metamorphosed Lower Coniston Limestone^ of 

 \V<(sdale Head. 



Drawn in polarized light to show the polysynthetic structure and zonary banding. 

 The + indicates the position oF the crossed nic^ls. Included granules 

 of pyroxene, &c. are omitted for the sake of distinctness. 



The existence of optical anomalies in the garnets of the Kalh- 

 silicathornfelsen has long been recognized. The property of double 

 refraction seems to be constantly accompanied by polysynthetic 

 twinning and a more or less pronounced zonary banding. Klein 

 has distinguished four different types of structure in the doubly 

 refracting garnets. Those of Wasdale Head belong, so far . as our 

 observations go, to his llhomhendodehaedertypus, which is defined 

 as built up by twelve hemimorphic rhombic pyramids, each having 

 its base on a face of the rhombic dodecahedron and its apex at the 

 centre of the crystal. The appearances actually seen in slices of the 

 rock of course vary considerably, according to the direction in which 

 each individual crystal chances to be cut. Rosenbusch * states 

 that the type of structure in question is by much the most common 

 in the doubly-refracting garnets, and gives an excellent figure in 

 illustration of it. In our specimens the division between the 

 several individuals of the polysynthetic twin is often rather irregular. 

 The zonary banding is usually, but not always, well marked. Kone 

 of the zones are isotropic, but the birefringence varies considerably 

 from zone to zone and is negative in character. When strongest, 

 it is about equal to that of quartz : only in rather thick slices do 

 the interference-tints rise to the yellow of the first order. Doubly- 

 refracting garnets, though not confined to metamorphosed limestones, 



* ' Mikr. Piiysiogr. d. petrogr. wicht» Miner.' 2nd ed. (1885) p. 264, and 

 pi. xiv. fig. 2. 



