424 



J. A. THILLIPS ON THE CHEMICAL AND MINERALOGICAL 



the same rock partially due to progressive alteration. In order, 

 however, to trace the nature and progress of the successive changes 

 which have occurred, a series of sections from typical specimens of 

 each variety has been cut and examined ; and in each case a portion 

 of the rock examined has been subjected to chemical analysis. 



Beginning with the fine-grained and least- altered rock from the 

 most westerly quarry, and arranging the specimens in accordance 

 with the apparent extent of their respective changes, the following 

 are the results of their several analyses. 



Table showing Composition of variously altered Roelcs from 

 the Penmaenmaicr Quarries. 



I. II. III. IY. 



w , f hygrometric... '12 *40 *05 traces. 



Water \ combined '95 1-82 264 4-46 



Silica 5845 6031 62-24 61-75 



Phosphoric anhydride trace trace trace trace 



Alumina 17-08 18-99 18-25 18-88 



Ferric oxide -76 1-07 105 -52 



persulphide , -39 '09 



Ferrous oxide 461 431 3-08 3'52 



Manganous oxide trace trace trace trace 



Lime 7*60 5"81 4-69 3-54 



Magnesia 515 '83 227 190 



Potassa 102 167 1-49 124 



Soda 425 455 379 3-67 



99-99 99-76 9994 99-57 



Specific gravity 2-94 2-79 275 2-79 



I. From the most westerly Quarry. — This rock, which is fine- 

 grained, is distinctly green in colour, and breaks with a conchoidal 

 fracture. Thin sections, when examined in ordinary light, appear 

 to be composed of a transparent base, in which are enclosed greenish 

 yellow but almost colourless crystals of hornblende, together with 

 indistinct crystals of felspar, a little magnetite or ilmenite, and a 

 few rare needles of apatite ; each section also usually contains an 

 occasional grain of crystalline quartz. When viewed in polarized 

 light, the transparent base breaks up into a brilliant mosaic of 

 crystals of triclinic felspar, of which the majority are about y-J^ of 

 an inch in length: many of these exhibit the characteristic striation 

 of plagioclase; but in the more slender crystals a single longitudinal 

 division is alone visible *. Sections of this rock, seen with crossed 

 prisms, afford typical examples of macrocrystalline structure. 



II. From the most easterly Quarry. — This rock, although mode- 

 rately fine-grained, is decidedly coarser than the foregoing, and is 

 less green in colour. When examined under the microscope, sections 

 are seen to consist of a crystalline aggregation of felspar, hornblende, 

 and quartz, through which are disseminated partially altered crystals 

 of ilmenite, together with, sometimes, a little iron pyrites. The 



* In one of the sections examined a group of felspathic crystals ^ of an inch 

 in length, which do not exhibit the structure of plagioclase, is enclosed in the 

 finely crystalline base. 



