150 



KNOWLEDGE. 



Aprii., 1911. 



the eastern half of the Isle of Skye. This Glenelj,' sheet, in 

 particular, is a splendid example of colour-printing. Nearly 

 sixty different ghades of colour (including hatched and 

 stippled patterns) are used, and a lartjc number of dykes and 

 sills are mapped, so that altogether the map is very complicated : 

 but nevertheless the use of colour-printing has made it 

 possible to issue the sheet at the low price of half-a-crown. 

 The issue of this sheet and memoir is opportune, as the 

 Geologists' Association (of London) will be visiting the district 

 this coming summer, under the guidance of one of the Survey 

 officers, Mr. .\lfred Harker. M.A., F.R.S., whose work on the 

 Tertiary Igneous Rocks of Skye is so justly celebrated. 



The ancient Lewisian and Moine rocks of the Glenelg area 

 are dealt with in detail in the memoir and some interesting 

 lithological types described, such as the brilliantly-coloured 

 eclogites and garnet-amphibolites, with a garnet-fuchsite rock 

 (containing the vivid green chrome-mica, fuchsite). Lewisian 

 limestones occur containing various silicates, especially 

 diopside, which sometimes forms masses up to several yards 

 across, also abundant forsterite crystals and great poikilitic 

 plates of phlogopite enclosing grains of calcite. As to the 

 puzzling Moine schists some evidence appears to indicate 

 that these were laid down iiiiconforiuably on the Lewisian 

 gneiss, and, while no definite conclusion is reached, various 

 facts suggest that they may be altered representatives of 

 Torridonian sediments. 



A large relief-model of Central Skye has been constructed, 

 which brings in most of the western half of this Glenelg sheet. 

 There are copies of this model at the Survey Museum (Jennyn 

 Street), at tlie Science Museum (South Kensington), at the 

 coming Coronation Exhibition, and elsewhere. 



The memoir on " The Geology of the Neighbourhood of 

 Edinburgh " was first published in 1S61. The new edition, the 

 second, forms a valuable guide to this interesting district. The 

 main geological interest lies, perhaps, in the igneous rocks, and 

 the petrography of these forms an important part of the 

 volume. Noteworthy are the numerous highly-alkaline types, 

 often containing analcite, as, for example, the associated 

 Essexites, Tesehenitesand Picrites, as well as Mugearite. This 

 last peculiar type, first described by Harker from the Tertiary 

 Series in Skye, and recorded in this memoir from the 

 Carboniferous of Edinburgh, has also been found in the 

 neighbourhood of Glasgow, while within the last few weeks 

 Mr. H. H. Thomas has proved its occurrence in the 

 Ordovician volcanic series of Skomer Island. Pembrokeshire 

 (Abstract of the Proceedings of the Geological Society. 

 February 1st, 1911). The main peculiarity of this type lies 

 in the paragenesis of olivine with such alkaline felspars as 

 oligoclase and even orthoclase. 



The structure and age of the old volcano of .Arthur's Seat. 

 Edinburgh, has long been a subject of contention. On the 

 one hand MacLaren, over seventy years ago, maintained that 

 it represented two entirely distinct series of volcanic outbursts, 

 separated by a vast interval of time, the older being of Lower 

 Carboniferous age. This interval was marked by the 

 deposition, subsequent upheaval and removal by denudation 

 of at least three thousand feet of Carboniferous strata. In the 

 first edition of the present memoir, Geikie followed this view 

 and considered the supposed younger volcanic series to be 

 probably of Tertiary age, but later he referred them to the 

 Mesozoic, and then to the Permian. Professor Judd. in 1875. 

 advocated the theory that the supposed second series of 

 volcanic outbursts had no existence, but that all the rocks are 

 the result of a single and almost continuous series of eruptions 

 confined to the Lower Carboniferous period. The evidence 

 obtained during the recent revision of Arthur's Seat by the 

 Geological Survey has confirmed Professor Judd's contention. 



ORIGINAL GNEISSOSE BANDING.— The term 

 " gneiss " covers a multitude of rocks, which differ vastly 

 not only in composition, but in mode of origin. Their only 

 community lies in the possession of that foliated character 

 known as the gneissose structure. Some have been derived 

 from sedimentary rocks (the paragnciss of Rosenbusch, 

 epigneiss of Reusch and mctagneiss of Lepsius) ; others are 

 of igneous origin iorthogneiss of Rosenbusch), while in others 



Hhi2 protogneiss of Lepsius) some see the primary crust of tlie 

 earth. Vet another type, the adergnciss of Sederholm, is both 

 igneous and sedimentary in origin, the foliation being the 

 result of the injection of many veins of pegmatite into a 

 sedimentary rock, so that the sediment becomes thoroughly 

 permeated by igneous material. This type is described among 

 the Lewisian rocks of tlie Glenelg area in the memoir already 

 referred to in these columns. 



But confining our attention to orthogneisses only, we find 

 several distinct types, of which the most usual has — at any 

 rate in the past — been regarded as due to regional meta- 

 morphism. Professor G. H. \Villiams was one of the early 

 exponents of the school which laid great stress on this mode 

 of origin. .Another possible means of producing foliation in 

 igneous rocks (of plutonic type) is by successive intrusions of 

 magma of different composition into the same consolidation 

 site. Thus Harker accounts for the coarser banding of tlie 

 peridotites of Rum in the Inner Hebrides. There remain two 

 very similar methods, the intrusion of one molten mass already 

 heterogeneous and the simultaneous intrusion of two different 

 magmas. The finer banding of the Rum peridotites and the 

 foliation of gabbro in the Cnillin Hills, Skye. have been 

 explained in this manner, as well as a perfect banding in the 

 dioritic complex of the Island of Orno. near Stockholm. 



The foliation in the Cortlandt Series — an igneous complex 

 ranging from granite through syenite, monzonite, diorite, 

 gabbro and norite to p^'roxenite and peridotite, occurring 

 about thirty-five miles north of New York City — has been 

 dealt with by several authorities. Professor J. D. Dana 

 regarded the rocks as worked-o%er sediments, \'olcanic ashes or 

 tuffs which, on being subjected to intense local inetamorphism, 

 lost most of their bedded structure and became pseudo-massi\ e; 

 later he treated them no longer as paragneisses but as of 

 igneous origin (orthogneisses), and Dr. G. H. Williams, in 1S86. 

 regarded the foliation as due to regional metamorphism. In 

 Tlie American Journal of Science for February, 1911, 

 G. S. Rogers demonstrates the origin of these rocks (where 

 norite and pyroxenite form alternate layers of constant grain) 

 to be by " magmatic differentiation. " as in the cases of the 

 Rum peridotites and the Skye gabbros. 



Up to the present this original gneissoid banding has not 

 been recognized in many localities ; it may, however, prove 

 illuminating (especialh' if found to be more common than is at 

 present thought) in connection with some of the puzzling 

 structures of the ancient and obscure igneous gneisses. 



THE HARDNESS OF MINERALS.— In spite of all 

 criticisms Mohs' scale of hardness still holds its place as the 

 standard of reference in Mineralogy. Breithaupt interpolated 

 two extra minerals between numbers 2 and 3 and 5 and 6 

 respectively, converting Mohs' scale of ten minerals into a scale 

 of twehe, in the attempt to make the intervals between 

 successive numbers more uniform. By various methods of 

 estimating "absolute hardness," Pfaft', Jaggar and others, 

 shewed that the intervals are far from uniform, and thus the 

 series of figures representing absolute hardness form a 

 progression which does not approximate either to arithmetical 

 or geometrical progression. But most noteworthy are Rosiwal's 

 figures, obtained in 1892, which show that Topaz (number 8 

 in Mohs' scale, where 1, the softest, is Talc, and 10, the 

 hardest, is Diamond) is softer than Quartz (number 7). Thus 

 in detail inversion of the correct order is indicated. 



Rosiwal obtained his figures by using a standard abrasive 

 to grind the mineral surface, and determining the loss of 

 weight suffered by the mineral when a given weight of the 

 abrasive was used up. In The American Journal of Science, 

 February, 1911, H. Z. Kip describes how he obtained a 

 similar result by a different method, which depends on the 

 force required to produce abrasion on the mineral, by a 

 diamond-point sclerometer. 



■■ Pfaff, Jaggar and others who .arrive at the opposite con- 

 clusion, have failed to eliminate the factor of density in 

 carrying out their tests. In other words, while regarding 

 hardness as resistance to abrasion, they have sought to 

 determine its value on the theory that it was to be measured 

 in terms of resistance to excavation." 



