Vol. 60.] THE LLYN-PADAKN DYKES. 379 



do not, in their unaltered state, show any sign of a second 

 generation of felspar; but in the crush-zone this structure, as 

 might be expected, is obliterated, the ophitic pyroxene becoming 

 granulitic, while the parts which have experienced the most 

 intense shearing have become almost schistose. In some parts, 

 albitization has gone on to such an extent, that the broken 

 pyroxenes are completely enclosed in large secondary felspars, 

 causing a complete reversal of the original structure, the pyroxene 

 then having the appearance of being the hrst-formed mineral. 



The general inference from all the slices is that consolidation 

 took place very slowly, probably under a thick cover of rock, 

 which may possibly explain the rarity of very marked sahlbands, 

 and certain cases of local enrichment in felspar, such as might 

 result from the concentration of this mineral, in accordance with 

 Soret's principle, owing to a prolonged duration of the liquid state. 

 This condition is still further indicated by the phenomena presented 

 by the augite, as will be more fully described later. 



The mineral-constituents of the rocks will now be described in 

 turn. 



Apatite. 



This mineral is present in conspicuous proportion in many of the 

 rocks of the Llyn-Padarn ridge area, but I have only occasionally 

 recognized it in the crush-zone. Mr. Harker mentions its general 

 occurrence in the Bala diabases. The capricious distribution of 

 this mineral in igneous rocks, and its usual immunity from any 

 marked effects of dynamic metamorphism, render it of little value 

 as an index to the amount of alteration which a rock containing it 

 may have undergone. For present purposes, therefore, it assumes 

 little or no petrographical importance. 



Iron-Ores. 



It will be convenient to consider the iron-ores next, although 

 these constituents did not entirely separate at any definite stage. 

 Some are idiomorphic, but they are also very commonly moulded on 

 the felspars and included in the augites. Generally speaking, 

 they agree so closely with Mr. Barker's description, 1 that it will 

 not be necessary to recapitulate these points. Titanic acid, how- 

 ever, appears to be more abundant in all the specimens from the 

 Llyn-Padarn dykes, and secondary alteration has resulted in a 

 large quantity of sphene-granules, in addition to amorphous 

 leucoxene. Mr. Harker noticed granular sphene in the Bala 

 diabase in one locality only, namely, at Pant-Evan, Tremadoc," 

 although he records its presence in some quantity in the 

 Llangwnadl rock, where the intrusion is presumably on a somewhat 

 lower horizon. On the other hand, no titanic acid was recognized 

 in the hornblende-picrite of Penarfynydd. It is also notably 

 absent in the later dolerite-dykes. In the light of these facts, the 



1 ' Bala Volcanic Series of Caernarvonshire ' 1889, p. 80. 



2 Ibid. p. 81. 



