Rev. 0. Fisher — Compression oj the Earth's Crust . 495 



analysis was employed : — After removing the silica according to 

 (a) a grain or two of pure ammonium cMoride is added to the 

 filtrate to prevent the precipitation of magnesium. It is then 

 heated to boiling, and a small excess of ammonia added, which 

 precipitates iron and alumina. They are determined together, and 

 then dissolved in the crucible with warm dilute hydrochloric acid. 

 The solution is treated with caustic potash, which precipitates the 

 iron and dissolves the alumina. The iron is filtered off and dis- 

 carded, because it cannot be thoroughly washed from the caustic 

 potash. The filtrate is slightly acidified with hydrochloric acid, 

 and the alumina is precipitated with freshly prepared ammonium 

 sulphide. The aluminum sulphide, when heated in a crucible, becomes 

 AI2 O3. The filtrate from the iron and alumina, containing the calcium 

 and magnesium, was heated to boiling and precipitated with a f 

 solution of ammonium oxalate, care being used to avoid much excess 

 of the reagent. The precipitate was allowed to stand eight or 

 twelve hours before filtering. The well-washed precipitate of 

 calcium oxalate, containing also a small quantity of magnesium oxalate, 

 was dissolved in warm, dilute hydrochloric acid, and the solution 

 was made alkaline with ammonia. This precipitates the calcium 

 oxalate, and leaves the magnesium in solution. This with the main 

 portion of the magnesium is precipitated as magnesium-ammonium 

 phosphate, and weighed as magnesium pyrophosphate. 



VII. — On the Cause of Compression of the Earth's Crust.' 



By the Rev. 0. Fishee, M.A., F.G.S. 



USED to think that the corrugations of the earth's crust were 

 due to compression through the shrinking of the interior. To 

 judge of the sufficiency of this cause the first thing to be done is 

 to seek a measure of the compression, and then to compare the 

 result of the effects of cooling with the actual amount of compression. 

 The most satisfactory measure appears to be the thickness of the 

 layer which the corrugations would form if levelled down. The 

 question then becomes one of how mucli. In 1863 Lord Kelvin 

 (then Sir W. Thomson) formulated a law of secular cooling upon 

 the hypothesis that the interior is solid. Adopting a probable 

 value for the contraction of rocks in cooling, I calculated the 

 thickness of the layer which would be produced by the corrugations 

 resulting, and found it far short of that which the existing inequalities 

 would form if levelled down. Mr. Mellard Eeade and Dr. Davison 

 subsequently discovered the existence of a level of no strain within 

 the crust, and this greatly reduces the possible amount of corrugations. 

 The conclusion at which I arrived was that, on the hypothesis of 

 a solid globe, secular contraction through cooling would not account 

 for the corrugations. 



Numerous phenomena suggest to the vulcanologist that the sub- 

 stratum is a liquid magma holding water-gas in solution. The free 



' Eead before the British Association, Cambridge, Section C (Geology), Aug., 1904, 



DECADE V. — VOL. I. NO. X. 29 



