GEOLOGY. 239 



Prof. Harkness entered into a somewhat elaborate comparison 

 of these deposits in Sweden, with the earliest fossil-bearing for- 

 mation in England, with the view of conveying some idea of the 

 immense antiquity of the former. We had now, he said, to pass 

 through 48,000 feet of strata from the existence of land plants 

 known up to the present period, to the existence of the land plants 

 which Mr. Torrell had made known to us. It would be impossible 

 to give any estimate of the time when these rocks were formed, — 

 it was too remote to be measured by human conception. When 

 we consider how slowly deposits take place, and the successive 

 strata of rocks which have come into existence since the plants in 

 question flourished, we can form some idea of the value of Mr. 

 TorrclPs discoverv. . 



These rocks were really at the basis of the Cambrian system, 

 or the primordial rocks of Barrande, and older than the oldest 

 rocks of Murchison's Silurian formation. 



FORMATION OF DENDRITES. 



Dr. Emerson Reynolds read a paper, before the Royal Geological 

 Society of Ireland, on the formation of dendrites. He had some 

 years since noticed that, when solutions of salts were placed upon 

 a plate of clean glass, and the glass placed between the poles of 

 a ilhumkorff coil, the salts gradually worked over the surface of 

 the glass in beautiful moss-like forms, which in many cases wxre 

 characteristic of the compound contained in solution ; the state of 

 dilution, at the same time, having some considerable influence. 

 The author proposed to call these " electric cohesion figures." To 

 produce them we will say that a drop of a solution of cyanide of 

 potassium is put in the centre of a phite of glass, which is then 

 placed upon a sheet of tin foil. One pole of the coil (it is imma- 

 terial which) is then brought into contact with the foil, and the 

 otlier pole is placed in the centre of the drop ; immediately on 

 passing the current the sohition begins to creep over the surface 

 of the glass in moss-like convolutions. 



The dendritic markings on minerals the author believed were 

 formed under a similar condition. He exhibited a beautiful man- 

 ganesous dendrite taken out of the museum. It was a conchoi- 

 dal limestone slab, and in his opinion illustrated this electrical 

 explanation conclusivel}'. Tiiere was orginally a flaw in the lime- 

 stone wliieh was exactly at right angles with the plane of cleavage. 



Through these flaws, as was evident by the marks, the mangane- 

 sous solution had percolated, and had perhaps ultimately been 

 the means of making the stone part in two, not however in the 

 direction of the flaws, but in the plane of cleavage. The den- 

 drites wliich w^ere formed upon the surface in this case were pro- 

 duced from the well-known fact that two surfaces at the instant of 

 their separation are in opposite electrical conditions. 



This phenomenon may be illustrated to a certain extent by in- 

 serting a drop of the fluid into the interstices of a plate of mica, 

 and then on suddenly parting the plate the dendritic fcjrms are 



