84 ME. E. H. WOETH OX THE [vol. lxxv, 



been exposed by quarrying, the next (LXXVI, S.E. 26) forms a 

 prominent outcrop. It is still finer in texture than 11, and quite 

 easy to section. Although it is as dark as the other rocks in hand- 

 specimens, the microscope shows much less pigment, and that in 

 discrete grains. There is an ill-defined wavy banding. The 

 ground-mass appears to be cryptocrystalline quartz ; in this are 

 set many rounded grains of a colourless isotropic mineral of high 

 refraction ; some of the grains show forms which may well be 

 sections of dodecahedra, their diameters varying from *07 mm. 

 downward, with rarely a somewhat larger form. Prolonged ignition 

 gives little glassy blebs on the surface, and it is fairly certain 

 these grains are garnet. Pale-brown mica in minute flakes is 

 scattered through the slide, and microscopic discrete grains of 

 iron pyrites are regularly distributed. The resemblance of 26 to 

 11 and to 1 c is obviously more apparent than real. The field- 

 evidence is all against 11 being a repetition of 1 c by folding, and 

 26 is evidently another species of rock. 



The more ordinary shales of the calcareous series vary in shade 

 from white to black, with green tints, and rarely brown, locally 

 developed near igneous intrusions. The colours often occur in 

 narrow bands, but are sometimes dappled ; the general effect may 

 be judged from the trade name of ' blue rock.' 



The conchoidal fracture is frequently masked by an imper- 

 sistent cleavage parallel to the bedding, and by an irregular 

 fracture across the bedding. In texture the rock is a porcellanite 

 rather than a calc-fiinta. Typical examples from the extreme 

 north and south of the belt (as, for- instance, from the Limestone 

 Quarry and from near Pedaven Mine) differ but little. 



The chief mineral in these rocks is undoubtedly wollastonite in 

 small laths and grains : the structure is much better seen in a 

 slide which has been treated with acid, and subsequently stained. 

 There is also a fair amount of quartz in irregularly interlocked 

 grains. Another colourless mineral occurs in small irregular 

 grains, generally somewhat elongated. This has a refractive 

 index higher than quartz and lower than wollastonite, while its 

 birefringence is higher than that of wollastonite. A rare larger 

 form of what appears to be the same mineral is evidently mica. 

 .Except in the presence of direct contact with an igneous dyke, 

 mica in clearly recognizable form and tourmaline are alike absent, 

 while rutile is never found even at the contacts ; these constitute 

 broad features of distinction from the aluminous series. 



All the porcellanites, even the whitest, contain a sufficiency of 

 iron sulphide to yield sulphuretted hydrogen and separated sulphur 

 on treatment with boiling hydrochloric acid. Possibly some of 

 the sulphide is in pyrrhotine. The acid solution, after treatment 

 of the powdered rock, contains a little iron, much lime, and appa- 

 rently no magnesia. 



- Under the influence of the main dyke of the Meldon aplite, and to 

 ajrainor degree at contacts with the smaller dykes, these calcareous 

 shales develop, in places, certain minerals in such quantity as to 



