Vol. 60.] THE LLYN-PADAKN DYKES. 383 



is often accepted. Neither this theory, however, nor that of Blum- 

 rich, 1 seems quite adequate to account for the phenomenon, which 

 appears rather to be a modification of zonary banding, and points to 

 a sequence of different phases during the formation of the crystal, 

 owing to changes in the conditions of equilibrium. In some cases 

 the lines of separation of the different portions correspond to lines 

 of crystalline growth, indicating mere pauses in growth, the next 

 accretion consisting of a new member in the series of isomorphous 

 mixtures. Generally, however, there was more than a pause. 

 Resorption began ; the salient angles of the last growth became 

 rounded off, and in some cases even greater corrosion took place, 

 before the crystalline growth was resumed in accordance with the 

 fresh conditions of equilibrium, which had, in the meantime, been 

 established. The difference in the extinction-angles of contiguous 

 areas reaches to as much as 10°, but is generally less. The 

 phenomenon is a very interesting illustration of the application of the 

 phase-rule in geology ; and if we accept Dr. Roozeboom's explana- 

 tion of the formation of mix-crystals, 2 it is possible that we may 

 find in this structure a proof of consolidation under variable 

 pressure, such as might occur in the case of a magma cooling 

 under the influence of earth-movements. The same 

 structure has been noticed in the Holyhead Main Dyke and in the 

 olivine-dolerite of Port Newry, 3 and I have also observed it in the 

 diabase of the Santon complex in the Isle of Man. 



The chief difference observed in the augites of the crush-area is 

 the development of mylonitic structures, the ophitic plates being 

 broken up into fragments, round which secondary albite has 

 crystallized. The fragments, however, exhibit the same poly- 

 somatic character, and have inclusions of the earlier pyroxenes as 

 described above. An intermediate condition, observed in some of 

 the specimens taken from the Llyn-Padarn ridge, near the southern 

 margin, has led to a very pronounced polysynthetic twinning in the 

 augites, often displaying two sets of twins crossing nearly at right 

 angles, and recalling similar strain-phenomena produced in metals. l 

 Such a difference in the effects of pressure upon the dykes enclosed 

 in the quartz-felsite and upon those in the sedimentary area is 

 very interesting, and resembles similar differences obtained 

 experimentally by Prof. F. D. Adams & Dr. J. T. Nicolson in marble 

 compressed under various conditions. 5 



Another effect of pressure-metamorphism, apparently related to 

 the above, is accompanied by a passage into amphiboles and 

 chlorites, to be described more" fully under these headings. 



I pass over the phenomena caused by simple weathering, as these 

 present no unusual features, and have no bearing upon the points 

 under discussion. I may, however, point out that this factor must 



1 ' Ueber die sogenannte Sanduhrform der Augite' Tschermak's Miu. u. 

 Petr. Mitth. n. s. vol. xiii (1893) p. 239. 



2 Zeitschr. f. physikal. Chem. vol. xxx (1899) p. 385. 



3 Geol. Mag. 1888, pp. 269 et seqq. 



4 J. A. Ewing & W. Eosenhain, Phil. Trans. Eoy. Soc. ser A, vol. exciii 

 (1900) p. 353. 



5 Ibid. vol. exev (1901) p. 3G3. 



