REPORT OF THE CHIEF ASTRONOMER 661 



SESSIONAL PAPER No. 25a 



the analogy breaks down; we shall see that, after all the lime salts are removed 

 from sea water, the ammonium carbonate has special power to attack the mag- 

 nesium salts. This fact cannot, for the reason just stated, be illustrated in the 

 case of the Black Sea. 



PRE-CAMlBRIAN SEDIMENTARY DEPOSITS. 



A third test of the hypothesis consists in an examination of actual rock- 

 deposits in the pre-Cambrian. 



Origin of Dolomite and of Other Magnesian Sediments. — It is an estab- 

 lished fact that dolomites and magnesian limestones, sandstones, and argillites 

 are very common in pre-Cambrian rock series. Granting that Eozoic organisms 

 could not secrete magnesium carbonate shells or skeletons, it follows that the 

 magnesian content of these rocks must have had a chemical origin. 



The magnesium carbonate was not thrown down simply because this little 

 soluble salt, as it was introduced by the pre-Cambrian rivers, saturated the sea 

 water. Then as now calcium carbonate was doubtless much in excess of magnesium 

 carbonate in river waters. We have seen that the acid radicals set free in the 

 persistent precipitation of calcium carbonate, would, during Eozoic time, prevent 

 the permanent solution of magnesium carbonate in any appreciable quantity. 

 On the other hand, we must conclude that the long-continued precipitation of 

 magnesium carbonate was effected by the action of a strongly alkaline carbonate. 

 We are thus naturally led to the discussion of the possible precipitation of 

 magnesium carbonate also by the ammonium carbonate emanating from decaying 

 animals. ' 



The experiments on this subject are, at first sight, contradictory. Linck 

 has recently shown that when ammonium carbonate is added to sea water, 

 aragonite is precipitated but no magnesium carbonate was found by him in the 

 precipitate.* On the other hand, Pfaff, using an artificial sea water similar in 

 composition to average sea water, found that, after a certain interval of time a 

 part of the magnesium in the salts was thrown down as the basic carbonate while 

 there was an abundant precipitation of calcium carbonate.! Pfaff's result 

 accords with the general experience of analytical chemists; hydrous magnesium 

 carbonate will be precipitated by the alkaline carbonate if time enough be 

 allowed. 



The reactions for magnesium sulphate and chloride commonly assume the 

 following forms : — 



4MgS0 4 + 4(NH 4 ) 2 C0 3 + H 2 = Mg 4 (CO :i ) 3 (OH) 2 + C0 2 + 4(NH 4 ) 2 S0 4 

 and 



4MgCl 2 + 4(NH 4 ) 2 C0 3 + H 2 = Mg 4 (C0 3 ) s (OH) 2 + C0 2 + 8NH 4 C1. 

 The presence of ammonium chloride tends to prevent the precipitation of the 

 carbonate from a solution of magnesium chloride; it is important, therefore, to 



* G. Linck, Neues Jahrbuch fiir Mineralogie, etc., Beilage Band, xvi, p. 502, 1903. 

 t F. W. Pfaff, Neues Jahrbuch fiir Mineralogie, etc., Beil. Bd., Voh ix. p. 504, 1894. 

 25a— vol. iii— 43£ 



