546 J. R. Dahyns 8f E. Oreenly — FelsHic Slates of Snotvdon. 



Cleavage and Metamorphism. 



Allusion has been made above, and was also in Mr. Dakyns' 

 paper of 1900, to the alteration which these rocks have undergone 

 in connection with the production of their cleavage ; and in order to 

 convey a clear idea of what they are now like it will be well to 

 describe this, especially as some of the phenomena are in themselves 

 of general interest. 



The body of the rock is now an exceedingly fine mosaic, giving 

 aggregate polarisation, but throughout this is a wonderful develop- 

 ment of white mica, fine indeed, but with some flakes quite large 

 enough to be recognised as individuals, and to give tints as high as 

 the first blue in a slide in which the felspars give nothing but low 

 greys. No part is free from this mica, but it is also aggregated into 

 anastomosing seams, of which some are as much as -5 mm. thick and 

 ■wholly composed of mica. Indeed, between crossed nicols the whole 

 slide lights up with its brilliant polarisation colours. There are 

 many rocks in regions of admitted crystalline schists in which the 

 development of mica is nothing like as great. 



There is also a good deal of chlorite and some grains of quartz, 

 and there are many lenticles of calcite, some of which reach 3 mm. 

 in length by '5 mm. in thickness. 



Most conspicuous are the crystals of felspar, many of which have 

 well-defined outlines, and often re-entering angles. Eound these 

 sweep the micas of the matrix, leaving the spaces under their ' lee ' 

 to which reference has been made. 



The micas, however, do not bend completely round them, for in 

 many cases they penetrate the crystal for some distance, its original 

 outline being then obliterated. This generally happens at the ends 

 of the augen, the micas which lie within the felspar crystal having 

 the same direction as those of the matrix, the direction, that is, of 

 the slaty cleavage. This is the same phenomenon as was described 

 some years ago by one of us from the Harlech Grits.^ In that case 

 the cause could not be indicated with certainty. Here it is evidently 

 an incident of the production of the cleavage. 



In the microscopical notes incorporated in Mr. Dakyns' paper of 

 1900, mention was made of certain bodies with igneous structure, 

 believed then to be fragments of igneous rocks. Most of them are 

 lenticular augen composed of intergrowths of felspars, with beautiful 

 polysynthetic twinning (Fig. 3). They lie in all directions within 

 the lenticle, and interlock at their edges. In some cases chlorite and 

 other products are also present, and it is possible that some of these 

 bodies may really be lapilli. But re-examination has shown that 

 a number of bodies, often with rectangular outlines, their longer 

 axes lying sometimes transverse to the cleavage, and quite in- 

 distinguishable in ordinary light from the larger original felspars 

 of the rock, reveal between crossed nicols the same internal structure 

 as these complex felspar augen, being composed of a mosaic of 

 clear, well-twinned felspars (Fig, 4). Some of the largest original 



1 Greenly, Trans. Edinburgh Geo!. Soc, vol. vii (1897), p. 251. 



