F. P. Mennell — Constitution of Igneous Roclis. 215 



Sock. Siiica, Area, Depth. Volume, 



per cent, square miles. . cubic miles. 



Granite . . 70 . 3,560 . 1 mile . 3,560 



Monzonite . 50 . 20 . ,, . 20 



Serpentine . 40 . 5 . ,, . 5 



Andesite, etc. 55 . 680 . 1,000 feet . 130 



Total , 3,715 



On this basis the silica average is 69'3 per cent., almost exactly what 

 is calculated for Rhodesia. Whether this is typical of Canada generally 

 I cannot say, but since one reads of granite masses a thousand miles in 

 length, it is probable that the silica percentage is not too high. 



These observations may be reinforced by another line of argument. 

 Mr. Clarke calculates the amount of quartz present in the igneous 

 rocks as averaging 12 per cent, of the whole. ^ He also gives estimates 

 of the average composition of the sediments, in this case, however, 

 making an assixmption as to the relative abundance of each class, and 

 merely giving composite analyses to obtain the composition of each 

 division taken separately. His figures for silica are: shales, 58-1 

 per cent. ; sandstones, 78"3 per cent. ; limestones, 5-19 per cent. For 

 quartz as apart from combined silica he gives: sandstones, 66'8 per 

 cent., and shales, 22"3, which are certainly not excessive estimates. 

 Now let us take Van Hise's assumption that, of the sediments, 65 per 

 cent, are shales, 30 per cent, sandstones, and 5 per cent, limestones.* 

 On this basis, even assuming that the limestones contain none, they 

 should average over 34 per cent, of the mineral. Clarke himself 

 assumes that the sediments comprise 80 per cent, of shales, 15 per 

 cent, of sandstones, and 5 per cent, of limestones. His figure for the 

 shales appears altogether excessive, and as the proportions he adopts 

 are chiefly based on his own calculations regarding the igneous rocks, 

 we should be merely reasoning in a circle if we accepted them. On 

 the basis of collating analysis, which he adopts for the igneous series, 

 the figures would be very different. Now there is only one source for 

 the sediments : they must all have been derived directly or indirectly 

 from igneous rocks, and if the latter only contained 12 per cent, of 

 quartz, as calculated by Clarke, how could the former contain 34 per 

 cent. ? No allowance for secondary quartz can overcome this difficulty, 

 for the obviously detrital quartz of the sandstones alone amounts to 

 20 per cent, of the whole bulk of all the classes of sediment taken 

 together. There is also another consideration which shows that 

 Clarke's igneous average allows far too much weight to the basic 

 division, namelj', the great deficiency of magnesia in the sediments. 

 On his estimate they should contain over 4 per cent., whereas they 

 actually contain less than 2^ on the most favourable assumptions. 

 The amount of potash also evidences the same tendency. 



In conclusion, I should like to pay a tribute to the value of the work 

 of which I have ventured to criticize one of the details. Mr. Clarke's 

 recently published volume should be of great service to geologists 



1 Loo. supra cit., p. 30. 



- Treatise on Metamorpliism : Mon. U.S.G.S., 1904, p. 940. 



