part 3] LOWER PALEOZOIC OP ARTHOG-DOLGELLEY. 301 



Felspar builds highly-idiornorphic crystals which yield rect- 

 angular, or sometimes almost quadrate cross-sections. Much of this 

 felspar appears to be orthoclase, but that some is albite is shown 

 by extinction-angles and refractive indices. These felspars usually 

 are slightly turbid with decomposition -products, and in a few 

 -cases they are coated with chlorite, which also accentuates their 

 •cleavage-traces. 



The felspar of the intergrowth shows a fine striation under high 

 powers, and it appears to be soda-orthoclase. Frequently this 

 intergrown felspar is in optical continuity with a narrow mantle 

 surrounding the idiomorphic plagioclases. 



Dark minerals are represented by chlorite in some slides, but in 

 others by rather ragged crystals of pale-green uralitic hornblende, 

 sometimes in the form of simple twins. Such fibrous hornblende- 

 crystals frequently show alteration to chlorite in the central parts, 

 and to blotches of brownish-green 'massive' hornblende in the 

 peripheral parts [C 275]. 



The chlorite is pierced by long narrow crystals of apatite, and 

 by scattered grains and rosettes of almost colourless epidote. Of 

 "the accessories, apatite at once strikes the attention by reason 

 of its abundance and size [C 275]. 



Iron-ores are also abundant, including magnetite in granular 

 aggregates and in perfect octahedra [C 275] ; also skeletal crystals 

 of ilmenite in all stages of replacement to leucoxene, and pyrites in 

 •cubes distributed sporadically. 



As already stated, rocks of markfieldite type pass upwards in the 

 larger Tyn-y-llwyn intrusion without break, into acid fine-grained 

 granophyre, or microgranite, of normal eurite type. At the actual 

 "top of the sill there is, however, a recurrence of basic material which 

 is for the greater part chilled. The chilled rock presents a porcel- 

 lanous appearance in hand-specimens, and under the microscope it 

 is seen to have lost all micrographic structures and to be composed 

 largely of acicular felspars [C 227; PI. XIX, fig. 6]. An acicular 

 habit of the felspar in marginal rocks was noted by Dr. A. Harker 

 in the Carrock-Fell granophyre. 1 



Genesis of the Granophyric Hocks. 



It is quite certain that all the varieties of granophyric rocks are 

 ■consanguineous, and that they were derived ultimately from a 

 common magma. In fact most of the types present are found 

 within a single mass, that of the larger Tyn-y-llwyn intrusion, in 

 which all transitions from thoroughly acid to almost basic rocks 

 are seen. The variety of types is due partly to deep-seated 

 differentiation, and partly to differentiation in place. The latter 

 process has evidently given rise to the narrow selvages of basic 

 material along both the upper and the lower contacts of the 

 •Crogenen granophyre. These selvages are too narrow (only 40 to 



1 Q. J. G. S. vol. li (1895) p. 127. 



