part 3] lower paleozoic of arthog-dolgelley. 291 



Microscopic characters. — The uniform character observed 

 in hand-specimens is seen again under the microscope, so far as 

 regards the mineralogical, and presumably therefore also the 

 chemical composition. The granophyre is everywhere a distinctly 

 .acid rock, consisting almost entirely of quartz and alkali-felspars. 

 Dark minerals are sometimes almost completely absent, and are 

 .always quite subordinate in amount, being usually represented only 

 by chlorite. In the marginal modifications, however, biotite and 

 hornblende make their appearance. 



The rock presents these same features throughout the length 

 .and breadth of the sill, the only changes being due to slight 

 variations in the texture. These variations are not, however, on 

 such a scale as to be appreciable in the field ; they are simply 

 variations in the degree to which micrographic intergrowths are 

 developed, and they may be observed within the limits of a single 

 slide. 



With regard to the characters of the minerals present, in examples 

 from the typical rocks the only porphyritic or microporphyritic 

 minerals are the felspars. Even they are not abundant, but each 

 :slide usually contains some examples which are generally clustered 

 together in groups of two or three highly-idiomorphic individuals, 

 sometimes partly surrounded by irregular plates of chlorite. Both 

 ■orthoclase and plagioclase are represented, the latter with albite 

 and pericline and less common types of twinning. One type, 

 though somewhat uncommon, appears to be characteristic of the 

 rock and its modifications. Cross-sections, perfectly square in 

 shape, show diametral twin planes which divide the section into 

 i'our equal squares ; each of these differs in interference from the 

 others (fig. 5, p. 292). Extinction- angles and refractive indices 

 ■correspond to an almost pure albite. In one case inclusions of 

 quartz-granules were observed. The orthoclase-crystals are about 

 the same size as the plagioclases, but they are not always truly idio- 

 morphic towards the ground-mass, showing that part of their growth 

 took place at quite a late stage. The late growth is further shown 

 by the manner in which chlorite has been pushed outwards by the 

 growing felspars. Both felspars are only moderately dusty-looking 

 in fresh specimens, and they do not possess the spongy structure 

 ■characteristic of the secondary albite in certain diabases ; they 

 may become filled with sericite, albite, and chlorite, but only as the 

 result of weathering. The microporphyritic felspars resemble the 

 felspar of the ground-mass in all respects, except in size. 



The ground-mass consists of quartz, felspar, and chlorites. In 

 normal specimens the quartz occurs as rather large grains which 

 interlock quite irregularly, and enclose small partry-idiomorphic 

 felspars. Whereas the quartz may act as a single crystal over 

 relatively large areas, the felspars, as a rule, behave quite indepen- 

 dently of each other. Thus the quartz acts as the host, and the 

 texture is micropoikilitic rather than micrographic. Such rocks 

 might perhaps be better described as microgranites rather than as 

 granophyres. 



Q. J. G. S. No. 303. x 



