424 ME. J. PAKKINSON ON" THE GEOLOGY OF [Aug. I9O3, 



the micaceous bands. These are often stained by a yellowish 

 pigment, which may possibly be of organic origin. In the lighter- 

 coloured bands the mica-flakes are sufficiently large to be indi- 

 vidualized, and are associated with small grains of quartz, 1 and 

 possibly of secondary felspar. These rocks, therefore, consisted of 

 argillaceous alternating with more arenaceous lamina?, in which, 

 through the agencies of heat and pressure, mica has been exten- 

 sively developed. 



Haematite and other iron-oxides characterize all the slides, 

 sometimes at least favouring certain bands in the rocks. Zircon, 

 rutile, and probably a little tourmaline are accessory minerals. 



Rusty-black bands, harder and darker than the silvery-grey soft 

 rock with which they are associated, are found in the railway- 

 cutting south of Hendraburnick, on the main road south of the 

 Rocky Valley, and elsewhere. A slight silvery sheen characterizes 

 the foliation-surfaces, athwart this direction a fibrous appearance, 

 the larger crystals being visible to the unaided eye. A thin section 

 shows the rock to consist of white mica-flakes, embedded in a matrix 

 of essentially the same composition, but in which the crystals are 

 too small and too closely entangled to be separated. A few grains 

 of haematite and a large number of orientated rods of micaceous 

 ilmenite 2 are scattered all over the slide. Quartz is practically 

 absent, tourmaline a rare accessory, and clinochlore occurs locally. 

 Numerous particles scattered through the slice are too small for 

 determination, but doubtless many are rutile. 



A thinly-bedded silvery to iron-grey phyllite, sufficiently soft to 

 be marked by the nail, and locally banded, which occurs at Trevivian 

 Farm, represents a common type in which clinochlore is absent. It 

 consists of closely-interwoven flakes of sericite, many of which can 

 be individualized between crossed nicols (*005 inch in length). 

 Doubtless some chlorite is also present ; and the slide is thickly 

 sprinkled with jagged grains of haematite roughly averaging 

 •001 inch across ; the brownish dust appearing under a high power 

 over a great part of the slice is possibly the same mineral. 



Multitudes of minute prisms of rutile 3 occur in the ' base ' of this 

 rock, and in varying degree in many of the preceding slides. 



In this type of rock without clinochlore minor variations occur 

 in the size of the flakes of white mica, the proportion of feebly- 

 polarizing chloritic scales, and the size and shape of the granules of 

 iron-oxide. 



The Penpethy Beds. 

 Excellent sections of these rocks are found on either side of the 



1 These rocks rather strongly recall the spilosites and desmosites of the 

 Harz. See J. J. H. Teall's ' British Petrography ' 1888, p. 218, and references 

 there given ; also A. Eenard, Bull. Mus. Rov. Hist. Nat. Belg. vol. iii (1884-85) 

 p. 233. 



2 Recorded by Mr. W. M. Hutchings, G-eol. Mag. 1889, p. 217. For descrip- 

 tion and figure, see A. Renard, Bull. Mus. Roy. Hist. Nat. Belg. vol. iii (1884-85) 

 p. 258 & pi. xiii, fig. 1. 



3 Recorded also by Mr. W. M. Hutchings, Geol. Mag. 1889, p. 220. 



