236 



PEOE. T. G. BONNET ON THE MICEOSCOPIC 



ing from the structure and optical characters, I should infer the 

 presence of a considerable quantity of the latter mineral, and so 

 name the rock a dolomite. 



51. (E. of Bodwrog, p. 218.) Is more coarsely crystalline with an 

 admixture of quartz grains and some silicate, which I am unable to 

 identify with certainty ; the carbonate, however, appears to be mainly 

 calcite. 



41. (At " a " of " Pentraeth," p. 213.) To the unaided eye appears 

 less crystalline and less pure, resembling some limestones which have 

 an admixture of volcanic mud. Under the microscope the calcite 

 appears in rather irregular grains of fair size and rather brecciated 

 aspect, imbedded in a streaky-looking paste composed of a pale green 

 serpentinous mineral, of black and dark brown dust, with a little 

 quartz, and of a clear silicate in minute granules, probably some 

 kind of zeolite. 



52. (Tau isa, N.E. of Cemmaes, p. 224.) Is moderately clear, 

 chiefly composed of very minute granules of crystalline calcite and 

 possibly dolomite, with occasional brown stains, and veined irre- 

 gularly with more coarsely c^stalline calcite. In no one specimen 

 can I detect any distinct trace of an organism ; but the first two 

 rocks differ much from the last two, which might readily pass for 

 members of the Carboniferous Limestone series. 



P. Igneous Rocks. 



39. (Fragment in green shale, E. of Bodorgan station, p. 214.) The 

 ground-mass is thickly crowded with small and not very sharply 

 defined felspar crystals, which, in their elongated form and the 

 twinning of larger specimens, generally resemble members of the 

 plagioclase group, interspersed with numerous minute grains of a 

 pale green colour, of which the larger are distinctly dichroic and are 

 probably hornblende. Small grains and occasional rod-like micro- 

 liths of an iron peroxide are also present. Scattered about in the 

 ground-mass are roundish patches of tessellated aspect, chiefly 

 composed of crystalline quartz ; one, in which the crystals are larger 

 and by their enclosures indicate lines of growth, has calcite at the 

 centre ; this, however, as in the case of a neighbouring vein, is 

 probably a subsequent infiltration. The majority more resemble 

 imperfectly formed spherulites ; a portion of the slide shows a 

 distinct fluidal structure, and the ground-mass has a general resem- 

 blance to several modern lavas, e. g. to specimens in my collection 

 from Astroni and the Solfatara (Phlegraean Eields). 



53. (Carreg-winllan, Pensarn, S.E. of Amlwch, p. 222.) Exhibits a 

 rather clear base, in which numerous small roundish patches are 

 defined by exceedingly minute opacite and ferrite, the latter some- 

 times forming the inner edge of the boundary ; it is also more or less 

 powdered about through the base. With crossing Nicols this last is 

 seen to be crowded with rather acicular felspar microliths, and the 

 majority of the patches are well-defined spherulites with the usual 

 radial structure. The rock is undoubtedly a trachyte, like the 

 other : and, I think it highly probable that each is from a lava-flow. 



