416 S. ALLPOET ON THE METAMOEPHIC EOCKS 



transverse section of a polygonal prism exhibits alternating bands of 

 blue and brown. 



Of this arrangement I have observed several examples in " schorl 

 rock" from Cape Cornwall; and there are excellent examples of blue 

 and brown tourmaline in one of the schorlaceous granite-veins near 

 Mousehole : the central portion consists of the usual schorl rock, 

 gradually passing into fine-grained granite on each side, the width 

 of the vein being about 14 inches. 



The mode of occurrence and the various crystalline forms of this 

 mineral present several points of interest. The well-known " schorl 

 rock," which occurs so frequently in the Cornish granite, consists of 

 a mass of crystalline granular quartz, in which distinct crystals of 

 tourmaline are often enclosed ; frequently, however, the latter has 

 no definite crystalline form, and then occupies the interstices be- 

 tween the grains and crystals of quartz, presenting the appearance 

 of having run in among them in the form of long straggling bands. 

 In such cases the tourmaline is not always in a granular state ; for 

 some of the long branching arms prove to be portions of a single 

 crystalline mass, the action of which on polarized light is uniform 

 over the whole surface. Although the assumption of a regular crys- 

 talline form may thus be prevented by the presence of other rigid 

 bodies in sufficient quantity, yet, on consolidation, the molecular 

 grouping of the elements is precisely the same as though no such 

 interference had taken place ; for, as we have seen, an irregular 

 shapeless mass may possess the same optical properties as a perfect 

 crystal. In specimens like the one here mentioned the tourmaline 

 occurs in comparatively small quantity, while in others it consti- 

 tutes a very considerable proportion of the mass, and then forms 

 more or less perfect crystals, in which quartz grains are enclosed. 

 Between these extremes there is every possible gradation. 



It is evident, therefore, that in some cases the quartz solidified 

 before the tourmaline ; for it has impressed its form on the latter ; 

 in other specimens, however, long prisms of schorl may be seen to 

 traverse one, or even several grains of quartz, which had evidently 

 not solidified when the prisms were enclosed. It appears, therefore, 

 from these facts, that when the schorl rock separated from the 

 granite the entire mass must have been in a plastic state, and that, 

 in accordance with varying conditions, either of the two minerals 

 might be the first to crystallize. 



In the granite itself the tourmaline and quartz present precisely 

 the same relations as in the schorl rock : sometimes both are inter- 

 stitial and appear to have crystallized together ; yet not unfrequently 

 the schorl must have been the first to consolidate. 



Small well-formed crystals frequently occur in the quartz of the 

 granite, of the schorl rock, and of the altered slates, and in many 

 cases are built up of minute acicular prisms, which project from one 

 end of the crystal, while the other is perfectly terminated, as seen in 

 PI. XXIII. figs. 8, 9, & 10. The specimen in which they occur 

 was broken off close to the granite, and consists of fragments of slate 

 and granite intermingled with quartz containing tourmaline and 



