REVIEWS. 



Geochronologische Stubien iJBER DIE Spatglaziale Zeit IK 

 SuDFiNNLAND. By Matti Sauramo. Bulletin de la Com- 

 mission Geologique de Finlande, No. 50. Helsingfors, 1918. 



rpHE extension into Finland of Professor De Geer's method of 

 -*- obtaining the time-values of the stages of glacial retreat is an 

 event well worthy of note. Dr. Sauramo 's memoir deals with a 

 considerable area of Southern Finland, both north and south of the 

 great Finnish end-moraine known as the Salpausselka. This region 

 was largely submerged beneath the Late-Glacial or Yoldia Sea, the 

 sedimentation in which was controlled by the flood waters from the 

 retreating ice-sheet. The summer floods brought coarse sandy 

 sediments ; the winter cold reduced the glacial outflow, and the finer 

 silts and clays were deposited at this season. There has resulted 

 an annual lamination, which, strange to say, is so remarkably con- 

 stant over wide areas that correlation from place to place is possible. 

 The m«tliod of correlation introduced by De Geer, and applied by 

 him with such spectacular success in Sweden, consists in drawing 

 graphs along the axes of which are marked equal intervals repre- 

 senting the years, and erecting ordinates proportional to the thick- 

 ness of the corresponding laminae. The graphs of the difierent 

 sections can then be compared by sliding them over one another 

 until a coincidence is obtained sufficiently remarkable to establish 

 identity. 



The distances over which Professor De Geer effected these 

 correlations were considerable, but it is more than surprising to learn 

 that they can be carried out with a fair amount of success by direct 

 comparison between sections some 200 miles apart on opposite 

 sides of the Baltic. Seven sections, which the author of the paper 

 under review ultimately fitted into the Finnish chronology, were sent 

 to De Geer without localities or other clue to their identification. 

 In the case of five of these he attempted a correlation with the 

 Swedish record, but failed in two instances, with regard to one of 

 which he had expressed doubt. The remaining three he placed in 

 the correct relation to one another, in which they ultimately worked 

 out in the Finnish record, and dated two of them with respect to the 

 third with an error of less than five years in a thousand. 



A study of the laminated clays corresponding to the stages of 

 halt in the retreat, the most important of which are those marked 

 by the inner and outer wreaths of the Salpausselka, has rendered 

 possible very accurate estimates of the duration of these stages, 

 and brought out clearly the fact that they are periods of minimum 

 summer ablation. The coarse summer zone, instead of forming the 

 greater proportion of each year's sedimentation, is reduced to a 

 negligible thickness or disappears altogether. The halt is thus 



