492 



UK. P. RTJTLEY 0~N THE ROCKS 



they pass in parallel lines across such lamellae without any accom- 

 panying twin-lamellation ; and in such cases they sometimes appear 

 as dark opaque rods, probably the edges of thin tabular crystals of 

 minute dimensions. 



These alterations are evidently similar to those described by 

 Prof. Judd under the term " Schillerization "* ; and it would 

 have been superfluous to figure them, since like examples are admi- 

 rably delineated in pi. x., vol. xli. of this Journal. By reflected 

 light some of these alteration-bands appear of a pale greyish-white 

 colour ; they are more opaque than the unaltered parts of the felspar, 

 and it seems highly probable that the minute scales, which in great 

 measure made up these bands, are kaolin. 



In one or two cases the felspar crystals appear to undergo parallel 

 extinction ; and bearing in mind the low extinction-angles of some 

 of the triclinic felspars, notably in basal sections of oligoclase and 

 andesine, I have felt some doubt in referring such crystals to ortho- 

 clase or to a micro-perthitic growth of that mineral with another 

 felspar. There appears to be no perceptible change of tint when a 

 Klein's plate is employed, and it seems possible that the very regular 

 parallel bands of decomposition-products may, in this case, be fol- 

 lowing a direction of cleavage in a monoclinic crystal. Due pre- 

 caution was of course taken to insure the accurate crossing of the 

 nicols. I am, however, still doubtful whether these crystals can 

 be regarded as orthoclase. Some of the felspars, indeed a large 

 proportion of them, appear, from their extinction-angles, to be labra- 

 dorite. Twinning on both the albite and pericline types may 

 sometimes be met with in the same crystal. Little or no quartz is 

 present, and the rock may be regarded mineralogically as diorite, 

 petrologically as a gneissic diorite or hornblendic gneiss. The 

 foliation of the rock is scarcely evident in the microscopic section, 

 except in one or two places where the smaller hornblende crystals 

 show a tendency to form short and irregular bands (PI. XIX. fig. 3). 

 The dioritic character of the rocks of the North Hill was duly 

 recognized more than twenty years ago by Dr. Harvey B. Holl f. 



No. 4. North Hill. Easternmost quarry, North end. — A very 

 coarsely crystalline rock consisting of blackish-green crystals of horn- 

 blende, ranging from a quarter of an inch in diameter to smaller 

 dimensions, and flesh-red felspar. This is one of the most coarsely 

 crystalline and profusely hornblendic rocks in the whole range. 



Under the microscope the larger proportion of the rock is seen to 

 consist of hornblende in large crystals, which by transmitted light 

 appear of a green colour. The pleochroism is strong and the 

 cleavages are well defined. Their boundaries are, however, irre- 

 gular, as are those of the felspar crystals. The latter have under- 

 gone much alteration, so that it is difficult to ascertain their optical 

 characters. Some of them, however, are plagioclastic and tolerably 



* " On the Tertiary and Older Peridotites of Scotland," Quart. Journ. Greol. 

 Soc. vol. xli. p. 376. 



t "On the G-eological Structure of the Malvern Hills" &c, Quart. Journ. 

 Geol. Soc. vol. xxi. p. 83. 



