﻿460 PROP. E. W. SKEATS ON THE GNEISSES [Aug. I9IO, 



boracic vapours or solutions with the ferromagnesian minerals of 

 the dacite. It is a curious circumstance that, up to the present, 

 tourmaline has not been noticed in the highly schistose or gneissic 

 rocks. 



In the altered dacites just described the banded or slightly- 

 schistose structure is more noticeable in the field than under the 

 microscope, although in some cases a parallel arrangement of the 

 minerals can be seen in thin sections. 



Highly altered dacites.— No. 426 (PI. XXXIV, fig. 1) may 

 be taken as an example of a rather more banded or schistose dacite, 

 in which, however, none of the effects — such as strain-polarization, 

 or granulitization of the phenocrysts — attributable to dynamic 

 metamorphism can be seen. There is, nevertheless, a noticeable 

 parallelism in the disposition of the minerals in the rock. This 

 may be in part due to an original flow-structure in the rock, or may 

 be of later origin. Certain structural and mineralogical changes, 

 such as the criss-cross aggregations of biotite and the marginal 

 corrosion of both biotite and hypersthene, are almost certainly 

 of secondary origin. Some of the felspar phenocrysts also show 

 minute granules of secondary felspars developed within them. All 

 these characters closely ally the rock with the less banded or 

 schistose dacites. 



(d) The Gneissic Hocks. 



In these rocks very extensive structural and mineralogical 

 changes have taken place. The field-evidence already adduced 

 shows clearly that the gneisses are not marginal modifications of 

 the granodiorite, but suggests strongly that they represent an ex- 

 treme stage of alteration of the dacites. In thin section these 

 rocks have a granular ground-mass of quartz and felspar similar to 

 that of the altered dacites described above. In both series the size of 

 the granules in the ground-mass shows variations in different rocks, 

 but the granules are larger in the gneisses than in the banded 

 dacites. 



No. 371 (PI. XXXIV, fig. 2) is a representative of the extreme 

 gneissic stage, from a locality south of Selby. Both structurally and 

 mineralogical] y this rock shows certain differences from the less 

 altered rocks. In some of the more altered dacites near the contact 

 the progressive replacement of ilmenite by biotite, and of hypersthene 

 by biotite and quartz, has been described. In the gneisses hypersthene 

 is entirely absent, and ilmenite is very rarely present. In their place 

 one notices an abnormal development of biotite. The proportion 

 of phenocrysts to ground-mass in the two types appears to be about 

 the same, and the total area occupied by biotite, hypersthene, and 

 ilmenite in the dacite is about the same as that of the biotite in 

 the gneiss. The plagioclase phenocrysts are still recognizable in the 



