22 



KNOWLEDGE. 



January. 1013. 



Relation to Oxidation ; and the very important Report on 

 Diffusion in Solids by Dr. C. H. Desch, in which the con- 

 clusion is drawn that " the occurrence of diffusion in metals 

 is established beyond any doubt ; but that experiments are 

 still lacking to prove its occurrence in transparent crystals of 

 minerals, salts, or organic substances, even under favourable 

 conditions, although, even here, indirect evidence points to its 

 possibility." 



Other notable papers are On the Dissociation of Phos- 

 phorus Vapour, by Professor Stock and Dr. Gibson, and 

 The Chemical Nature of Uranium X, Radio-Actinium 

 and Thorium B by Mr. Fleck, who was unable by means of 

 fractional precipitation to effect any concentration of the 

 short-lived radio-element in thorium, or to separate thorium 

 from radio-actinium or thorium B from lead. 



GEOLOGY. 



By G. W. Tyrrell, A.K.C.Sc, F.G.S. 



GEOLOGY OF THE LIZARD AND MENEAGE — A 

 memoir with the above title has just been published by the 

 Geological Survey, and written by Dr. J. S. Flett and Mr. J. B. 

 Hill. The Lizard has long been a happy hunting ground 

 for geologists, largely on account of its fascinating petrological 

 problems, especially those connected with the great mass of 

 serpentine which is the central feature of its geology. 



The Lizard area can be divided into a northern and a 

 southern portion. The dominant feature of the northern 

 portion is the occurrence of four bands of sedimentary rocks 

 (killas or clay slate, with subordinate grits, limestones, and 

 conglomerates), named the Mylor, Falmouth, Portscatho, and 

 Veryan series respectively, of which the last-named is the 

 youngest, and has been determined by fossil evidence to be 

 approximately of Llandeilo or Arenig age. Volcanic activity 

 in Veryan times gave rise to the well-known spilitic lavas 

 of Mullion Island, which are interbedded with limestones and 

 radiolarian cherts. 



The southern area is composed of a great metamorphic 

 series, consisting of sedimentary and igneous rocks, of which 

 the latter are overwhelmingly predominant. The metamor- 

 phosed sedimentary rocks are the oldest, and consist of 

 mica-schists, granulites, and green schists, containing also 

 hornblende schists of undoubted igneous origin. Subsequent 

 to the formation of these rocks, an intrusion of granite took 

 place. This rock has been greatly metamorphosed, and is now 

 a highly foliated hornblende gneiss, which occupies the Man 

 of War Islands off the Lizard shore. 



Before the intrusion of the great serpentine another sedi- 

 mentary formation was deposited, the Treleague quartzite, a 

 quartzose rock in which the original pebbly structure is well 

 preserved. In many places also, on the margin of the 

 serpentine, there are coarse, gnarled, hornblende schists — 

 the Traboe schists — representing the coarse dolerites or 

 gabbros which immediately preceded the great plutonic 

 intrusions. 



The serpentine covers an area of twenty-one square miles, 

 and is probably the largest serpentine mass in the British 

 Islands. It has a more or less circular outline and is clearly 

 a large laccolite or boss similar to the granite bosses that 

 stud Cornwall and Devon. Three main varieties have been 

 distinguished, the cherzolite or bastite-serpentine, the 

 tremolite-serpentine, and the dunite serpentine. The first 

 named of these is the rock which affords the beautiful 

 ornamental stone for which the Lizard serpentine area is 

 famous. 



Later the serpentine was invaded by some bosses and an 

 enormous number of dykes of gabbro. In many localities 

 these have been crushed and rolled out with the formation of 

 " flaser" -gabbros. After the cooling of the gabbro, a further 

 injection of basic material took place, giving rise to abundant 

 black dykes of olivine-dolerite, many of which have been 

 crushed into epidiorites and hornblende-schists. Before this 

 episode had finished, a final uprush, this time of acid material, 

 occurred ; and in some places, a mixed or hybrid rock, con- 



sisting of imperfectly mingled acid and basic material was 

 produced, forming a heterogeneous banded gneiss called the 

 Kennack Gneiss. 



It is interesting to remark how this long and intricate 

 igneous period of the Lizard is paralleled in the Ordovician of 

 Ayrshire. Here also is a large mass of serpentine, intruded 

 by gabbro and dolerite dykes with later masses of acid 

 material. These intrude Ordovician sediments, the basal 

 portions of which contain numerous flows of spilitic lavas 

 associated with limestone and radiolarian cherts just as in the 

 Mullion Island district of Cornwall. 



THE PETROLOGY OF SANDSTONES.— The petro- 

 logical study of the sedimentary rocks is now an important 

 adjunct to their stratigraphical study. The investigation of 

 the constituent grains of a sandstone is often especially 

 helpful in obtaining an idea of the mode of origin and the 

 derivation of the rock. Important results are being obtained 

 from the study of the Scottish Carboniferous sandstones. An 

 investigation by Mr. T. O. Bosworth, brought forward at the 

 British Association, shows that these sandstones belong to 

 two entirely different kinds, those in which the heavier 

 mineral grains consist mainly or largely of garnet, and those 

 in which garnet is absent or scarce. As far as Mr. Bosworth 

 has yet examined the Carboniferous succession, the Coal 

 Measure sandstones have been found to be highly garneti- 

 ferous, whilst those of the underlying Millstone grit were 

 almost entirely non-garnetiferous. In the sandstones of the 

 Carboniferous Limestone Series, out of fifteen samples 

 examined, nine were garnetiferous, and six devoid of garnet. 

 The Calciferous Sandstones were found to be entirely non- 

 garnetiferous. 



The garnets, and indeed all the heavy grains, were found 

 by Mr. Bosworth to be characteristically angular. The 

 garnets were broken along the dodecahedral cleavages, giving 

 elaborate zig-zag shapes with numerous corners and edges. 

 These grains were in marked contrast to those found in desert 

 sands. 



An independent investigation by Mr. W. R. Smellie (Tran- 

 sactions of the Glasgow Geological Society, 1912) of the 

 Upper Red Barren Measure sandstones, which overlie the 

 Coal Measures to the east of Glasgow, shows that the sand- 

 stones suffer a progressive change upwards in regard to 

 mineral content and the degree of rounding. The lower beds 

 are still characterised by angular garnets, but these do not 

 occur to nearly the same extent as in the Coal Measure sand- 

 stones. The higher beds, however, are devoid of garnet, and 

 contain abundant zircon, rutile, and tourmaline, the grains of 

 which are frequently well-rounded and polished, especially 

 the zircons. 



MICROSCOPY. 



By F.R.M.S. 



LOW POWER PHOTO-MICROGRAPHY.— BACK- 

 GROUNDS. — It is a practical maxim in picture making of 

 all kinds, scientific, technical, and pictorial, that nothing is 

 seen without a background. 



Thus a uniformly lighted quite white object is unnoticeable 

 against a uniformly lighted equally white background. 



The next point is that the useful effect of a background is 

 to show contrast. This is so obvious that that it is very 

 frequently entirely ignored. In Figure 20 we have a pre- 

 cisely similar pair of tiny cowrie shells side by side, similarly 

 lighted. That on the right is backed by black paper, that on 

 the left by a bit of white postcard ; the two backgrounds 

 being pasted side by side on an ordinary micro slip so that 

 both parts had the same lighting, exposure and development. 



First we notice that the shell on the right seems to be 

 decidedly lighter than that on the left. But this is an optical 

 delusion due to contrast effects with the backgrounds. Next 

 we notice that with the light background (left) we get a 

 decided cast shadow as well as a shaded side, and that where 

 this side of the shell is close to the light card background 

 we get a little reflected light on the shadow side, while with the 



