Vol. 60.] THE LLTN-rADARX DYKES. 381 



seen in isolated instances. A prevalent combination shows one 

 half of a Carlsbad twin simple, and the other half with the albite- 

 lamellation. The usual form is in long laths, and in the uncrushed 

 parts the extinctions are sharp. On sections oriented in the zone 

 100 on 001 the extinction-angle generally exceeds 20°, indicating 

 a predominance of the anorthite-molecule. This is significant, as 

 the post-Carboniferous dolerites have usually a rather less basic 

 felspar, and generally show a second generation of a more acid 

 species, with zoriary banding. In the rocks that I have examined 

 from this area, two generations of felspar are apparently not 

 present, and zonary banding is rarely exhibited. This fully agrees 

 with the characters shown by the felspars in the Bala diabases. 



In proportion to the amount of crushing that the rocks have 

 undergone, characteristic changes are noticeable in the felspar, the 

 most striking of which are the secondary felspars, often conspi- 

 cuously present in large water-clear crystals, with ill-defined outlines, 

 and sometimes showing shadowy twin-structures. Where such 

 ' albitization ' has taken place, the remnants of the older felspars 

 are easily distinguished by their extensive saussuritization, bent 

 outlines, corroded margins, undulose extinction, and by being often 

 included in the later secondary crystals. The secondary albites 

 also include epidote, viridite, and broken pyroxenes, while here and 

 there the characteristic ' felspar-mosaic ' of Lossen is exhibited. It 

 is difficult to measure the extinction-angles of these secondary 

 felspars, suitable crystallographic planes being wanting. In some 

 cases, however, it is possible to compare by Becke's method the 

 refractive index of the felspar with that of an adjacent crystal of 

 secondary quartz. The result agrees with the refraction of albite. 



All these characters are very typical of sheared diabases, and 

 indicate considerable pressure-metamorphism. Their importance 

 in the present discussion lies in the proof which they afford that 

 the rocks have been subjected to extensive earth-move- 

 ments. They are not, so far as I am aware, the characters 

 usually exhibited by the felspars of the later dykes of Caernarvon- 

 shire ; neither does Mr. Harker mention them as occurring in the 

 Bala diabases. But the specimens described by him were apparently 

 not so much crushed, and were collected from areas more remote 

 from the Llyn-Padarn ridge. 



The saussurite and other alteration-products of the felspar show 

 no unusual features. The large quantity of pale epidote and calcite- 

 dust is an additional evidence of a considerable lime-percentage ; 

 and all the phenomena go to show that the original magma wa.- 

 of a typically-basic composition, and that the separating felspars 

 belonged to the lime-end of the albite-anorthite series. The original 

 composition of such a felspar, however, may be easily obscured by 

 secondary changes leading to the break-up of the anorthite-mole- 

 cules, and their replacement by epidote, calcite, and quartz. Such 

 changes may be traced along the course of these dykes, isolated 

 specimens of which, if taken from the crush-zone, would seldom 

 give an adequate clue to their original composition. As before 



