338 DR. J. A. SMYTHE ON 



The two are very similar, thus bearing out the petrological 

 likeness of the rocks and rendering it probable that they are 

 weathered in the same manner. 



The determination of some constituents in the Hartley 

 rock, soluble in dilute acid, is given in Table D. 



Table D. Constituents of the weathered Hartley Dyke, soluble in 



dilute acid. The sample analysed is the same as in Table C, No. I. 



Fe (ferrous) 2*43 



Fe (ferric) o - 8i 



CaO 17-36 



MgO 0-83 



Comparing this with the analysis in Table C, No. I., some 

 interesting points come to light. Firstly, the attack by 

 weathering agencies on the augite is so complete that what 

 little magnesia remains in the rock is almost wholly in a 

 condition readily soluble in dilute acid ; secondly, as the 

 amount of carbon dioxide, i3 # 66 per cent., is equivalent to 

 17*36 per cent, of calcium oxide, and as this is the amount 

 actually found, it may be concluded that the carbon dioxide 

 exists entirely combined with lime in the form of calcite, and 

 thus 4/27 per cent, of lime (that is 2i*63-i7'36) is still present 

 in the form of lime-felspars. This amount corresponds closely 

 to that found in the Crookdene rock, namely 5*4 per cent. 

 Thirdly, it may be noted that the ratio of ferrous to ferric iron 

 in the acid-soluble portion is as 3 to 1 : but as it may be 

 safely assumed, judging from many analyses of similar dyke- 

 rocks, that the ratio in the fresh unweathered rock is 

 approximately as 1 to 1, it follows either that the weathering 

 agent has had a reducing action, or, what seems more likely, 

 that it has attacked the ferrous silicates preferentially, pro- 

 ducing thereby soluble ferrous compounds and leaving un- 

 altered the ferric silicates. 



The question now arises as to the nature of the agent which 

 has been operative in weathering these dykes, and it would 

 seem that calcium bicarbonate, Ca (HC0 3 ) 2 , is the only one 

 which could bring about these changes in the augite, re- 

 placing, at the same time, the leached-out minerals with 



