NATURE 



THURSDAY, JUNE 5, 18S4 



THE ORIGIN OF THE CRYSTALLINE 

 SCHISTS 



Untersuchungen iiber die Entstehung der Altkrystal- 

 linischen S chief ergesteine mit besonderer Bezugnahme 

 auf das Sachsische Granulitgebirge, &"c." von Dr. 

 Johannes Lehmann. (Bonn: Hochgiirtel, 1884.) 



NO problem in modern geology stands out with such 

 prominence as the origin of that remarkable group 

 of rocks to which the name of the Crystalline Schists has 

 been given, and to none in recent years has so large a 

 share of the literature of the science been devoted. The 

 question is attacked on all sides. By some observers its 

 solution is sought in laborious investigations of the hilly 

 and mountainous regions where these rocks hold their 

 empire among the grander solitudes of nature. By others 

 the question is studied in the quiet of their own libraries 

 or laboratories with all the resources of modern chemistry 

 and microscopy. Great progress has indeed been made 

 in these various ways. Regarding certain aspects of the 

 problem a general agreement has been arrived at ; but 

 there are others as to which the difficulties remain as 

 persistently obstructive as ever. 



One of the most important contributions to the study 

 of this fascinating subject has just appeared in the form 

 of a handsome quarto volume, with a large atlas of plates, 

 by Dr. J. Lehmann. This author has enjoyed excep- 

 tional opportunities of qualifying himself for the task he 

 has now undertaken. For nine years, as a member of the 

 Geological Survey of Saxony, he was engaged in the 

 investigation of the classical granulite-region of that 

 kingdom, of which he surveyed the southern and rather 

 larger half, while his colleague, Mr. E. Dathe, investi- 

 gated the northern part. The maps of this Survey are 

 accompanied with explanatory pamphlets, among which 

 Dr. Lehmann's detailed local observations have already 

 been published. But it was desirable to present a 

 generalised description of the whole region and to discuss 

 the bearings of the observations upon theoretical ques- 

 tions. He originally proposed to undertake this task in 

 association with Mr. Dathe ; but his transference to Bonn 

 as Privat-docent in Mineralogy and Geology, and the 

 removal of his friend to the Prussian Geological Survey 

 having prevented the intended cooperation of the two 

 observers, Dr. Lehmann has himself worked up the mass 

 of materials collected during his long course of work in 

 the field. To enlarge the scope of his inquiries and 

 obtain additional data for comparison he has recently 

 extended his investigations into the Erzgebirge, Fichtel- 

 gebirge, and the mountains of the Bavarian and Bohemian 

 frontier. And he now offers what we may hope is only a 

 first instalment of his results. 



Naumann, whose early account will always be cited 

 as a model of careful observation and accurate descrip- 

 tion, regarded the granulite of Saxony as an eruptive 

 rock — an opinion in which he has still modern followers, 

 including our author himself. He recognised a fact which 

 seems in more recent times to have been lost sight of, 

 Vol. xxx. — No. 762 



that a gradation can be traced from the more highly 

 crystalline condition of the granulite centre, through 

 successive zones of mica-schist, and other schists, into 

 the older sedimentary rocks of the surrounding dis- 

 tricts. These schistose rocks have in more recent times 

 been classed as "Archaean," and as such they appear on 

 the maps of the Saxon Geological Survey, Dr. Lehmann 

 having himself accepted this view in his earlier published 

 descriptions. But more extended study of the subject 

 has induced him to abandon the idea of the existence of 

 any Archaean nucleus and to return to a modification of the 

 original conception of Naumann. How he has been led 

 to this conclusion it is the object of his volume and atlas 

 to show. 



Under the deep cover of post-Tertiary deposits, the 

 granulite tract of Saxony forms a central ellipse round 

 which zones of various schistose rocks are grouped, that 

 pass outwards into the normal clay-slates of that part of 

 German}-. These slates on the south-eastern margin are 

 unconformably overlaid by Silurian and Carboniferous 

 rocks. On the north-west side a conformable sequence 

 is traceable from the schists and slates upwards into 

 Cambrian and Lower Silurian rocks, which are precisely 

 like those of the adjacent countries. Instead of being 

 Archaean masses, Dr. Lehmann concludes that the whole 

 of the crystalline schists within the granulite area are 

 metamorphosed Palaeozoic sediments. They may be 

 originally of Silurian or Cambrian age, and their meta- 

 morphism probably took place during the crumpling and 

 upheaval of the area, that is, later than the Devonian and 

 older than the Carboniferous period. 



Towards the establishment of this conclusion the author 

 brings forward a vast mass of detail, which he skifully 

 arranges so that its bearings upon theoretical questions may 

 be clearly seen. At the same time he endeavours to sepa- 

 rate rigidly what is demonstrable fact from what is mere 

 inference, and in this lies one of the most valuable features 

 of his memoir. He has collected such a body of evidence 

 as will give a new impetus to the study of metamorphism, 

 while at the same time it provides abundant new and 

 suggestive material for the prosecution of this study. He 

 justly cites the Saxon granulite area as a classic example 

 of the occurrence and origin of metamorphic schists where 

 acomplete gradation can be followed from unaltered or little 

 altered sediments into wholly crystalline foliated masses. 

 In this progressive intensity of metamorphism the most 

 notable fact is the corresponding advance in the develop- 

 ment of mica. Over and above all local diversities of 

 mineral character, there is a constant augmentation in the 

 quantity and size of the mica-folia. At the same time the 

 muscovite, which is alone present in the outer parts of the 

 area, is replaced further inwards by biotite. Nor is this 

 change confined to the peripheral schists ; it extends into 

 the granulite of the centre. Such a rearrangement of the 

 mineral constituents of the rocks cannot be explained by 

 any hypothesis of an eruptive granitic mass. Like so 

 many other concurrent facts, it points to the effects of the 

 molecular movements of the original rocks, sedimentary 

 or other, under the strain to which they were subjected 

 during the process of crumpling and upheaval. Where 

 these movements have been greatest, there the accom- 

 panying metamorphism has been most intense, and, as 

 one prominent indication of this change, there is the 



