NA TURE 



\_Junc 5, 1884 



most abundant development of biotite. Every student 

 of the crystalline schists can furnish parallel examples 

 to those cited by Dr. Lehmann where, on the zigzag 

 puckerings that form so striking a feature among these 

 rocks, a copious growth of biotite or some other mica has 

 taken place. 



Among the metamorphosed rocks of the Saxon region 

 some of the most instructive are bands of conglomerate 

 interstratified among the schists. The sedimentary origin 

 of these zones is of course unquestionable, and so obvious 

 that the alteration to which they have been exposed 

 furnishes a kind of sample of the initial stages of change 

 which are so often lost where the clastic materials are of 

 a less prominent and obdurate character. The pebbles of 

 granite, quartz, &c. have been deformed and more or less 

 altered, so that sometimes they seem to shade off into the 

 surrounding matrix. The latter has become a crystalline 

 micaceous mass by which the pebbles are wrapped round. 

 These conglomerate bands have thus been converted into 

 half-crystalline gneiss-like schists. 



A specially important part of the memoir deals with 

 Gabbros and Amphibolites. These rocks, as members of 

 the series of crystalline schists, have long been a puzzle to 

 those who have studied them in the field. That they are 

 metamorphic rocks, and not rocks of original chemical 

 precipitation, has been inferred from their association with 

 masses whose original sedimentary origin admits of no 

 doubt. But even those who have held this view have 

 hesitated as to the nature of the original masses out of 

 which they have come. Many years ago Jukes suggested 

 that hornblende-rocks and hornblende-schists might 

 represent ancient lavas and tuft's interstratified with the 

 sediments which are now schists and quartzites. And it 

 seems probable that this opinion is essentially correct. 

 Dr. Lehmann goes into great detail regarding the struc- 

 ture of the diallage and hornblendic rocks of the granulite 

 tract. His study of them leads him to conclude that the 

 gabbro is an eruptive rock, younger than the granulite 

 but elder than the granite, which has been involved in 

 the general metamorphism and has consequently assumed 

 schistose modifications. " 1 know no rock,'' he adds, 

 " which illustrates so well the effect of mechanical pres- 

 sure upon a solid rock as the gabbro of the Saxon granu- 

 lite tract. While other rocks leave us in doubt as to their 

 original condition, the gabbro supplies us with every stage 

 from the beginning to the end of the metamorphism." 

 These conclusions possess at present a special interest in 

 relation to the crystalline schists of this country. The 

 Geological Survey, in the course of an investigation of 

 the schists of the north of Scotland, has recently come 

 independently to similar deductions with regard to the 

 diorites and amphibolites of Aberdeenshire and Banff- 

 shire. Among the schists of that region there occur ex- 

 tensive masses of diorite. This rock presents sometimes 

 the typical composition and structure of a diorite, and 

 under the microscope appears as one of the most beauti- 

 ful examples of a thoroughly crystalline granitoid mass. 

 It behaves in the field as an eruptive rock, which has risen 

 generally parallel with, but also transgressive across, the 

 bedding of the contiguous schists. It is obviously from these 

 1 ters a mass that has been intruded into the clay-slates, 

 knotted-schists, and other schists of the district. Being 

 traversed by veins and bosses of granite, its protrusion 



was obviously earlier than that of some at least of the 

 granite. Further examination of it, however, shows that 

 in many places it presents a remarkable parallelism in the 

 arrangement of its crystalline constituents. Sometimes 

 this is shown by the orientation of the feldspars in one 

 definite direction. In other places the feldspar and horn- 

 blende are drawn out into more or less distinct bands. 

 Further stages of change reveal the feldspar segregated 

 into an almost pure labradorite rock, while the hornblende 

 appears as a felted mass of hornblende-schist. Some of 

 these schistose aggregates are of exquisite beauty. Over 

 wide tracts biotite has been abundantly developed in the 

 diorite, and sometimes also numerous and large kernels 

 of garnet. It is observable that the direction of the folia- 

 tion of the diorite coincides with that of the surrounding 

 schists. There seems no reason to doubt that, as these 

 Scottish schists are metamorphosed Lower Silurian sedi- 

 ments, the diorites and amphibolite-schists represent 

 Palaeozoic eruptive rocks that have participated in the 

 general metamorphism. Dr. Lehmann recognises, in the 

 Mica-schist and Phyllite groups, hornblende-schists which 

 he thinks may have been embedded masses of diabase 

 that have been more or less altered. 



His general conclusions are thus summed up : — " I can- 

 not regard the metamorphic schists (mica-schists, gneisses, 

 Sec.) as 'Archaean ' formations. It does not appear to me 

 to be established that genuine gneisses anywhere came 

 out of pre-Cambrian sediments. The production of such 

 rocks as mica-schist, &c, belongs to the time of mountain- 

 upheaval, and in actual fact has involved formations ot 

 far younger age than the Cambrian. In the Saxon 

 granulite region it is later than the Devonian period." 

 He draws a distinction between what he considers to be 

 " true gneisses " and other rocks to which the general 

 name of gneiss has been applied. He restricts the appel- 

 lation to the foliated forms of granite. This foliated or 

 true gneissic structure he believes to be more or less due 

 to metamorphism by stretching, seldom wholly original, 

 so that many gneisses may be called metamorphic ; only, 

 the original nick was not a sediment but a mass that con- 

 solidated from fusion (Erstarrungsgestein). We fear that 

 a theoretical distinction of this kind will involve all kinds 

 of practical difficulties in its general application. 



Reference must be made to the atlas that accompanies 

 the memoir. It contains 28 plates, on which are placed 

 no fewer than 159 photographs of thin sections of the 

 rocks described in the text. L T nlike the usual illustra- 

 tions of this kind, these photographs represent the objects 

 of the natural size, or less, or at most only slightly mag- 

 nified. They are not microscopic studies, but show the 

 actual structure of the rocks as seen by the naked eye 

 or with a weak lens. It is impossible to speak too 

 highly of the success with which they have been 

 produced. With their aid we are rendered in some 

 measure independent of the actual specimens, and 

 can follow with pleasure and satisfaction the detailed 

 descriptions of the author. No such wealth of accurate 

 illustrations has yet been furnished for the study of this 

 important series of rocks. Dr. Lehmann, however, is, we 

 hope, only on the threshold of his inquiries. A vast 

 domain lies before him where the problems are many and 

 the qualified observers are but few. He has done excel- 

 lent service by presenting in this compendious form such 



