166 J. A. PHILLIPS ON THE SO-CALLED 



II. Roseliill, Castle Hornech. — This rock is brown in colour, and 

 is more fissile than the foregoing, consisting largely of crystalline 

 hornblende, through which hydrated ferric oxide and a chloritic 

 mineral are disseminated ; it includes patches of some nearly colour- 

 less translucent material, into which minute green crystals project 

 from the circumference *. 



III. Chyandour, Penzance. — This is a very hard rock, dark in 

 colour, frequently breaking with a subconchoidal fracture, and in 

 which nearly all trace of cleavage or lamination has disappeared ; 

 when freshly broken its surface is sometimes spotted with patches 

 of the size of small shot of brown crystalline hornblende. Thin 

 sections are found to be composed of small interlaced acicular 

 crystals, through which a flocculent mineral of a darker green 

 colour is disseminated. 



IY. Rosemorran, Gulval. — A distinctly lamellar rock of the usual 

 dark green colour, and to a considerable extent fissile. Thin sec- 

 tions are seen to consist of a felted aggregation of a fibrous horn- 

 blendic mineral, in which are imbedded a few larger crystals of 

 hornblende. Irregular minute patches of granular quartz are scat- 

 tered throughout the mass ; these, which contain liquid-cavities, are 

 sometimes milky, and at others transparent, being traversed in all 

 directions by belonitic crystals, which penetrate from the exterior. 

 A few nebulous stains of viridite and hydrated oxide of iron, together 

 with occasional needles of apatite, are disseminated throughout. 



It will be observed that the foregoing specimens of slaty rock, 

 obtained from localities at considerable distances apart, closely 

 resemble one another in general chemical composition, and are 

 almost identical in their penological characteristics. They contain, 

 on an average, ten per cent, less silica than the more highly 

 crystalline rocks of which analyses are given p. 162 ; and a small 

 proportion of phosphoric anhydride is present in all of them. 



The microscopic closely felted crystals, of which these rocks are 

 often to a considerable extent composed, seldom exhibit distinct 

 outlines, and not unfrequently assume a more or less flocculent 

 appearance. 



This mineral, which is dichroic, is believed to be hornblende, 

 although the low percentage of silica found by analysis might appear 

 to indicate the presence of chlorite. When we take into considera- 

 tion, however, the small proportion of combined water in these 

 rocks, it becomes apparent that if chlorite be present the amount 

 must be very small f . 



* All the rocks belonging to this class are to a large extent decomposed by 

 boiling hydrochloric acid ; but on placing an unmounted section from this 

 locality on a glass slip under the microscope, and covering it with hot acid, 

 some of the lighter patches were observed to be more readily attacked than the 

 remainder of the mass. In one or two cases a few minute bubbles of carbonic 

 anhydride were evolved, but the quantity was too small for estimation by 

 analysis. 



t The proportion of silica in hornblende usually varies between 40 and 50 

 per cent. ; but according to Bammelsberg (' Handbuch der Mineralchemie,' 

 pp. 492, 493) a black crystal of this mineral from Filipstad, Warmland, con- 



