168 J. A. PHILLIPS ON THE SO-CALLED 



This is a fissile and distinctly lamellar rock, which is employed 

 for road-making. Although its prevailing tint is grey, it is spotted 

 with patches of light brown hornblende mixed with a little greenish 

 mica. An examination of thin sections shows it to consist largely 

 of greyish microlites, so thickly matted together in a transparent 

 base as to render the general mass to some extent opaque. In this 

 mixture are patches of brown hornblende, a litttle mica, tremolite, 

 magnetite, and a few spots of iron pyrites : the magnetite is not 

 generally crystalline; and when seen in polarized light the base 

 exhibits no indication of crystalline structure. This rock contains 

 a few distinct markings having a resemblance to cloudy crystals of 

 plagioclase seen in certain varieties of dolerite. 



The foregoing examination of rocks forming the shores of Mount's 

 Bay indicates that they principally belong to three distinct series, 

 differing alike in their chemical composition and mineralogical con- 

 stitution : — 1st, augitic rocks of a class generally believed to be of 

 igneous origin ; 2ndly, ordinary clay-slates ; 3rdly, hornblendic 

 slaty rocks. 



The rocks included in the first of these divisions appear to have 

 been originally composed of felspar, augite, magnetite, and apatite ; 

 all have, however, in a greater or less degree, been subjected to 

 influences through the agency of which the crystalline character of 

 the felspar has to some extent been destroyed, while viridite and 

 hornblende have been produced by the alteration of augite. In 

 some instances these secondary changes have reached such an 

 advanced stage that the felspar is either represented by pseudo- 

 morphic forms, or has become transformed into an amorphous 

 or granular base ; in other cases the augite has been completely 

 removed, giving rise by its decomposition to pseudomorphs of which 

 hornblende or viridite are the chief constituents. In such specimens 

 apatite sometimes appears to be the only original mineral which has 

 not suffered from alteration. That these changes have, to a great 

 extent, been produced by a rearrangement of the ultimate con- 

 stituents of the rock, rather than by their removal and substitution, 

 is probable from the results of analyses of variously altered 

 specimens. 



The rocks of the second class are the ordinary clay-slates or 

 killas of the Cornish mines. 



The green lamellar rocks constituting the third class appear to 

 present evidences of sedimentary origin, and may have been pro- 

 duced either by the metamorphism of other rocks subsequent to 

 consolidation ; or, when in the state of paste, magma, or mud, they 

 may have become consolidated under such chemical and physical 

 conditions as to lead to the generation and crystallization of the 

 various minerals which they enclose. Reference to the table, 

 p. 165, shows that they not only possess a great resemblance to 

 one another in chemical composition, but also that in this respect 

 they more nearly approach the crystallized igneous rocks than 

 ordinary clay-slates ; the proportion of silica present, however, is 

 considerably less than is usually contained in hornblendic slates. 

 Cape-Cornwall District. — The so-called greenstones of Cape 



