334 Correspondence — Mr. U. Hill. 



attention to some places in Sweden where this denuding power also 

 could be traced. Prof. Torell also mentioned the marine fauna at 

 Gothenburg, and Mr. Bortzell made some remarks on the deep-boring 

 at Gothenburg. 



GeOLOGISKA FoRENINGEISrS FoRHANDLINGAR. STOCKHOLM. JSTo. 32. CONTENTS. 



1. Meeting. April 2, 1876. 



2. C. W. Blomstrand. Bidrag till kannedomen cm Langbansgrufvans mineralier. 

 (Contribution to the Knowledge of the Minerals of the Langban-mine in Sweden.) 



3. A. M. Jenrsfcrom. Om Finlands postglaciala skalgrusbaddan. (On the Post- 

 glacial Shell-beds in Finland.) ^ 



4. E. Erdmann. Profil genom en rullstensas. (Section of a Swedish " As " — 

 Karnes, Eskers — consisting of stratified sand and gravel with boulders.) 



5. Swedish, Norwegian, Finlandic, and Danish geological literature published 

 during 1875. 



APPARENT AND TEUE DIP. 



Si"R, — Mr. Penning's very simple construction for direction of 

 dip, given in the May Number of this Magazine, is only approx- 

 imate, and in extreme cases might be rather widely wrong. In the 

 case he has taken for his figure the error will be about 3° ; in the 

 case of dips of 30° and 60° at right angles to each other, his con- 

 struction would be 8° wrong. 



The accurate rule would be as follows : — 



Draw tv;o lines from a point in directions of the two observed 

 dips. Set oif along each line lengths proportional to the tangent 

 of the other dip (or to the cotangent of its own). Join the ends of 

 these lengths : the joining line will be the direction of the strike, 

 the perpendicular to it from the angle will be the direction of the dip. 



Moreover, if cotangents have been used, the length of this perpen- 

 dicular is proportional to the cotangent of the true dip, whose 

 magnitude as well as direction will thus be determined by one 

 construction. 



Mr. Penning's construction would give the magnitude of the dip 

 by the formula : Length of perpendicular : length of a side : *, dip 

 in direction of other side : true dip. And as accurate values of dips 

 are usually needless, this would probably give sufficiently correct 

 results. 



Mr. Penning's rule, as any one acquainted with Trigonometry 

 may see, is equivalent to assuming that angles are proportional to 

 their tangents. E. Hill. 



Cambridge, Ma^ 2drd, 1876. 



SOUTH AUSTRALIAN POST-TERTIARY FORAMINIFERA AND 



OSTRACODA. 

 SiE, — A notice of the occurrence of the following species of Fora- 

 minifera and Ostracoda in the Post-Tertiary beds of South Australia 

 may be of interest to some of your readers. A small collection of 

 Mollusca was presented to the Museum of Practical Geology, London, 

 many years ago, by Mr. H. Challoner, of the Burra-Burra Mines, 

 South Australia. From the matrix contained in and surrounding 

 these the minute fossils were washed by Mr. J. Bennie and myself. 



