212 F. P. Mennell — Const if uiion of Igneous Rocks. 



Our Natural History Society induced the Government in 1894 to 

 set aside a reserve for the preservation of the indigenous fauna and 

 flora, containing some 25 square miles in the Darling llanges, but 

 this was cancelled early last year. However, Barrow Island was 

 gazetted for this purpose in November last. It is situated about 

 60 miles off the north-west coast, and is about 14 miles in length 

 with an average width of 4 miles. It is the home of four species 

 of mammals and one bird {Malurus edouarcli) not found on the 

 mainland. Eernier and Dorre Islands on the north-west coast are 

 doomed now that the Government has set aside those islands as 

 hospitals for sick aboriginals. Vigorous efEorts are being made to 

 induce our Government to follow the enlightened policy of that of 

 the U.S.A., otherwise the majority of the marsupials will soon 

 become as extinct as the dodo and great auk. 



Y. — The Constitutiojt of the Igneous Eocks. 

 ByF. P. Mennell, F.G.S. 



IN a paper read at the 1903 meeting of the British Association and 

 afterwards published in this Magazine,^ the method of calculating 

 the chemical constitution of the igneous rocks by the collation of 

 analyses was criticized, and the results were compared with those 

 based on actual field-work in an area where such rocks were very 

 abundant. Mr. F. W. Clarke, whose results and those of Mr. Harker - 

 had interested me in the problem, has recently published a fresh calcu- 

 lation of the results of the rock analyses made in the Laboratory of 

 the United States Geological Survey. It is not proposed to discuss 

 these results in detail, and it will suffice for my present purpose to 

 take the question of silica percentage alone. Mr. Clarke's average 

 for all the 1,358 rocks analyzed up to the end of 1903 works out at 

 60'91.^ Mr. Clarke, however, admits that the ' salic ' rocks are more 

 abundant than the other classes,* and he further grants that my 

 criticism of his previous results is entitled to some weight, though he 

 thinks that a wider range of observation would probably modify my 

 opinion that granite is practically the average igneous rock. 



I have already insisted on the fact that to obtain trustworthy data 

 calculations must be based on areas where plutonic rocks are abundantly 

 exposed. It is absurd, for example, to give flows of basalt covering 

 the surface to a limited depth the same weight in oiir estimates as 

 granite masses which extend indefinitely downward. Tet this is the 

 result to which the collation of analyses leads us. Indeed, it goes 

 much further, for the composition of the great granite masses is so 

 readily ascertained upon mere inspection of microscope sections, or 

 even of hand-specimens, that they are rarely analyzed at all unless 

 they show exceptional features. And in getting at reliable averages 

 it is just these exceptional features that we must avoid. It is easy to 



1 Geol. Mag., 1904, Dec. V, Vol. I, pp. 263-4. 



- Geol. Mag., 1899, Dec. IV, Vol. VI, pp. 220-2. 



3 Data of Geo. Chemistry, Bull. 330, U.S. Geol. Siut., 1908, p. 26. 



* Loc. cit., p. 25. 



