August 30, 1877] 



NATURE 



381 



characters. If it has no double refraction, only one well-defined 

 circular hole can be seen. If the mineral possess double refrac- 

 tion and only one optic axis, like calcite, two images of the hole 

 are seen. If the section be cut perpendicular to the axis, 

 two circular holes are seen directly superimposed, but at two 

 different foci. If the section be in the plane of cleavage, two 

 widely-divided images are visible, the one due to the ordinary 

 ray being circular, and the other, due to the extraordinary ray 

 being distorted and drawn out in two opposite planes at two 

 different foci. When the section is cut paiallel to the axis, this 

 image due to the extraordinary ray is still more elongated, but 

 the images are directly superimposed. We thus at once learn 

 that the mineral has double refraction, has an optic axis, and also 

 what is the direction in which the section is cut. In the case of 

 crystals like Arragonite, which have two optic axes, there is no 

 ordinary ray, and at the focal points we see the circular hole 

 drawn out in opposite planes into crosses. The character of these 

 crosses depends upon the direction of the section, but the fact 

 of the crosses beuig seen at once proves that the mineral has two 

 optic axes. Some facts are better observed if, instead of a cir- 

 cular hole, we examine through the crystalline plate a grating 

 with two systems of lines at right angles to one another. We 

 then obtain what the author calls unifocal or bifocal images, 

 according to the systems of crystallisation. Crystals without 

 doxible refraction have only one unifocal image ; crystals having 

 one optic axis have one unifocal and one bifocal image ; whereas 

 crystals having two optic axes give two bifocal images. The 

 definition of unifocal images is independent of the position of 

 the lines, whereas in the case of bifocal images the lines are 

 distinctly visible only when they are parallel or perpendicular to 

 a particular axis of the crystal, and, spread out, become obscure 

 and disappear when rotated to a different azimuth. 



The above-named general characters difler so much in dif- 

 ferent minerals, that they furnish a most valuable means for their 

 identification. 



On the ' ' Great Flat Lode " South of Redruth and Camborne, 

 by C. Le Neve Foster, B. A,, D. Sc. — In this paper the author 

 described an important tin lode which is wrought in various 

 places for a distance of three and a half miles. In some places 

 it occurs, for instance at Wheal Uny, at the junction of the 

 clayslate (killas) and granite, but in other mines it lies entirely 

 in granite. 



Its characteristics are : — 



1. A /((!i!tr of true fissure vein, ''generally only a few inches 

 wide, and filled with clay, fragments of the inclosing recks, and 

 tin or copper ores, dipping 30° to 50° S., and striking from 20° 

 to 45° N. or E. (true). 



2. The lode, from four to fifteen feet wide, on one or both sides 

 of the leader, consisting mainly of schorl-rock, containing gr.\ins 

 and veins of tin ore. It yields from i to 3 per cent, tin ore. 



3. A capel, or non-stanniferous or slightly stanniferous 

 schorl-rock, separating the lode from the killas or granite. 



4. Absence of any loall or plane of separation between the 

 lode and capel, or between the capel and granite. The author 

 said that all the appearances pointed to the fact that the lode 

 and capel are merely altered granite. In confirmation of this vew 

 he explained that he had found cavities in the lode resembling 

 felspar crj'stals in shape, and probably left by its removal ; 

 furthermore the microscopic examination of the capel shows 

 apparently pseudomorphs of quartz after felspar. 



If it is admitted that the mass of the " Great Flat Lode " and 

 its capels are altered rocks once containing felspar, we are driven 

 to conclude that lliat rock must once have been granite, because 

 of the gradual passage of the capel into gianiie. Supposing 

 this view to be correct, we must adopt a similar explanation 

 in the case of many of the important tin lodes in Cornwall. 



The author ventured the opinion that half the tin ore obtained 

 in Cornwall is now derived from altered granite. 



On some Tin Mines in the Parish of IVcndron, Corinualt, by 

 C. Le Neve Foster, B.A., D.Sc. — The author described the tin 

 deposits of the following mines : — Balmynheer,the Lovell, and 

 South Wendron. The author supposes that the tinny rock is an 

 altered granite, and he brings forward in support of his argument 

 the fact that pseudomorphs of quartz and of gilbertite after ortho- 

 clase, are found in the stuff from the Lovell, and that there is a 

 gradual passage from the tin rock into granite. 



On some of the Stockworks oj Cormuall, by C. Le Neve 

 Foster, B.A., D.Sc— The author divided the tin stockworks into 

 three classes according as they occur in killas granite or elvan, 

 and then described the mode of occurrence of tin ore at|some of 

 the most important. 



The Carboniferous Limestone and Millstone Grit in the Country 

 around Llangollen, by G. H. Morton. — The author described 

 the carboniferous limestone exposed in the Eglwyseg ridge near 

 Llangollen, North Wales. He stated that the finest section is 

 exposed at the Ty-nant ravine on the bed of Cefn-y Fedw, and 

 that the country around must be considered as the typical area 

 of the lower carboniferous series of North Wales. The millstone 

 grit or Cefn-y-Fedw sandstone, which reposes on the limestone 

 in the same district, was also described. The following tabula- 

 tion explains the succession and thickness of the entire series : 



Tabular Vieio of the Carboniferous Limestone and Cefn-y-Fidw 

 Sandstone in the Country around Llangollen. 



Aqueduct grit or upper 

 sandstone and conglo- 

 merate 



Upper shale 



Dee Bridge sandstone ... 

 Cefn-y-Fedw ( Lower shale with fire-clay 

 sandstone. and bands of limestone. 



i Middle sandstone 



j Cherty shale 



Lower sandstone and con- 



i glomerate 



Sandy limestone 



Carboniferous 

 'limestone. 



Upper grey limestone 

 „ white ,, 



Lower ,, 

 ,,^ broBn 



feet. 



I Upper Cefn- 

 y-Fedw, Dee 

 70 . Bridge, or 

 30 millstone grit 

 30 J series. 



480 I 

 1923 



Middle and 

 lower Cefn-y- 

 Fedw or 

 Yoredale 

 series. 



Upper car- 

 boniferous 

 limestone. 



Upper old red sandstone 300 

 The following table shows the gradual attenuation of the 

 carboniferous limestone towards the south-east. 



Attenuation of the Carboniferous Limestone. 



c. K.I- :-■ , T . .. * Tan-y- Trevor Bron- i^ 



Subdivision. ly-nane. ^.^^^^^^ ^^^^^ ^^^.^^^ Fron. 



Upper grey limestone| 300 300 250 65' 8S' 



,, white ,, 300 250 140 99 27- 



Loner ,, ,, 120 115 117 104 ,, 



,, brown ,, 4S0 360 100^ 26-^ ,, 



I2C0 1025 607 295 115 

 This section shows how the limestone diminishes in thickness 

 with the rise of the Wenlock shale towards the south-east. 

 Between the Ty-nant ravine and Tan-y-Castell it has thinned out 

 200 feet, and at Fron-y-Cysyllte, four miles from the former 

 place, the attenuation is not less than 900 feet. 



The list of fossils collected by the author contained seventy- 

 seven species. Of these fifty-eight occur in the upper grey lime- 

 stone and only eighteen in the lower brown limestone. If the 

 carboniferous limestone is simply divided into upper and lower 

 limestone, thirty-eight species are peculiar to the two upper sub- 

 divisions and nineteen to the two lower subdivisions — twenty 

 species being comr ion to both. However, the species are by no 

 means confined to the subdivisions in which they -ire found near 

 Llangollen, for they occur at different horizons in other districts. 

 On the Occurrence of Branchipus or Chirocephalus in a Fossil 

 State in the Upper Fart of the Flitvio- Marine Series (Middle 

 Eocene), at Gurnet and Thorness Bays, near Cowes, Isle of IVight, 

 by Henry Woodward, F. R.S.— Mr. Woodward referred to the 

 great interest surrounding the geology of the Isle of Wight from 

 the labours of Ibbetson, Forbes, Mantell, Prestwich, Bristow, 

 and many others, and the rich fauna contained in its strata, 

 much of which still remains to be described, although the strati- 

 graphical geology has been well worked out by the oflicers of 

 the Geological Survey. Mr. Woodward called attention to a 

 thin band of freshwater limestone occurring [at the base of the 

 clilf, belonging to the Bembridge series, from two to twelve 

 inches thick, which at places is full of remains of plants and 

 insects. Dytiscus, Curctclio, Formica, &c, , and what is most 

 remarkable, the diaphanous bodies of a small phyllopod crusta- 

 cean, without a hard shelly covering. This little crustacean*is 

 closely related to the " Brine-shrimp " (Artemia salina), so 



^ Upper portions been denuded. 

 - Reposes on the Wenlock shale. 

 3 Base not ascertained with certainty. i 



