234 



PROF. T. Gr. BONNET ON THE MICROSCOPIC 



B. Chlorite-Schists. 



A group of highly altered, distinctly foliated rocks, consisting 

 mainly of rather minutely crystalline chlorite and quartz. To this 

 belong JSTos. 35-37, 45, 46, 49. 



35. (Porth yr Ogof, Holyhead, p. 226.) Contains very numerous 

 rods and granules of a black mineral, perhaps magnetite, with a little 

 sphene or staurolite ; it exhibits beautiful contortions. 



36. (Porth felin, Holyhead, p. 226.) Foliation less marked ; a con- 

 siderable amount of ferrite is present. Original structure perhaps 

 less uniform than in the last. 



37. (Porth y defaid, p. 225.) The same remark applies to this speci- 

 men also, but its foliation is more marked. A vein of quartz and calcite. 



45. (Melin pant y gwyda, p. 222.) The same ; chloritic constituent 

 rather minute. 



46. (Cas Clock, p. 222.) Highly altered and markedly foliated. 

 49. (Crogan goch, p. 223.) The same ; chloritic or possibly mica- 

 ceous constituent extremely minute ; beautifully foliated and contorted. 



In this group, as a rule, it is more difficult to identify with 

 probability any original constituent of the rock, owing probably to 

 differences in chemical composition and size of the materials. Some 

 of the quartz grains in 37, 45, 46 may, however, be original. 



C. Slaty and other not highly altered Rods. 



In this group there has, indeed, been a certain amount of altera- 

 tion, but it is only what may be termed micromineralogical. The 

 original fragmental character of the rock is still distinct ; the newer 

 products are either extremely minute or such as result rather from 

 decomposition than from recomposition. The alteration, in short, is 

 of a kind which we commonly meet with in the earlier Cambrian or 

 latest Prccambrian rocks, not in the gneisses, in the conspicuously 

 foliated schists, and in the most highly metamorphosed quartzites. Its 

 members, then, are, as a rule, either much more modern than those 

 previously described, or selective metamorphism must have operated 

 upon the latter to a rather unusual extent. 



43. (S.E. of Ccrrig ddwyffordd, pp. 214, 232.) One of the flinty 

 argillites or indurated imperfectly cleaved slates. Under the micro- 

 scope the slide consists of a brownish-grey earthy-looking material 

 interspersed with minute specks of quartz, felspar, and micaceous or 

 chloritic minerals, whether original constituents or of secondary origin 

 it is in many cases difficult to say. The " stripe," however, is well 

 indicated, and the specimen in no important respect differs from 

 many examples that I have examined from Charnwood and other 

 localities where the rocks are approximately of Cambrian age. 



44. ("W.S.W. of Cerrig Ceinwen, pp. 215, 232.) A similar rock 

 of slightly coarser material and a little more distinctly cleaved. 

 Among the constituents are man*y scales of a strongly dichroic green 

 mineral with a marked wavy cleavage — chlorite or some member 

 of the provisional viridite group, probably the result of alteration 

 of a magnesia-iron mica fragmentally present. 



47. (Llanfechell, p. 223.) A coarser fragmental rock with a rather 



