322 MB . ALFRED HABKER ON THE GABBRO [Aug. 1 894, 



of this, short of actual estimation of the iron, is the density of the 

 rocks. 



The specific gravities have been determined by the hydrostatic 

 balance on specimens usually of more than 50 grams, and so large 

 enough to eliminate small variations : the figures are all reduced 

 to 4° C. The mean specific gravity of specimens from forty-two 

 different localities in the gabbro area is 2-953, and the range of 

 variation is about 20 per cent, of this mean, the extreme figures 

 found being 2*679 and 3-265. 



If we take a traverse from north to south across the gabbro, we 

 find the specific gravity to decrease steadily until the central zone 

 is passed, and then to increase steadily to the other border of the 

 intrusion. Below are the figures for three parallel traverses across 

 the eastern portion of the area, where exposures are most frequent. 

 Each hiatus represents a place where the rocks are concealed : — 



3-222 3-265 3-200 



2-848 



2-804 2-850 2-933 

 2-778 2-822 2-800 

 2-844 2-890 2-872 

 2-877 2-939 2-922' 

 3-110 3-103 



It will be seen that, without exception, the gabbro grows denser 

 from centre to margin in both directions. The full significance of 

 the figures, however, is seen only when they are laid down on a 

 map. With a sufficiently large number of observations, it would 

 be possible to connect points corresponding to equal specific gravities 

 by lines like the contour-lines round a hill or the isobars round a 

 cyclonic centre. I have not attempted to go so far as this, but I 

 consider that the figures given are sufficient to establish the continuity 

 of the whole mass and the distribution of the several types in 

 roughly concentric zones becoming denser from the centre to the 

 margin of the area. The accompanying map (PI. XYI.) shows the 

 approximate course of lines corresponding to specific gravities 2-85 

 and 2-95. These arbitrarily chosen limits divide the gabbro area 

 into three parts : 



(i) A central portion of specific gravity less than 2*85 : here the 

 rocks are relatively acid, and usually contain rather abundant- 

 quartz ; 

 (ii) An intermediate zone of specific gravity 2-85 to 2-95 : con- 

 sisting of more normal gabbro, in which quartz is at most 

 an accessory constituent ; 

 (iii) A marginal zone of specific gravity above 2-95, and in the 

 limit very much higher: the conspicuous feature here is 

 the great abundance of the iron ores. 



Without entering into further detail, it will be taken as proved 

 that the various types are only parts of a single body of rock, which 

 becomes progressively richer in iron oxides (and, as we shall see, 

 in certain other constituents) from centre to margin, and that this 

 change is most rapid as we approach the actual boundary of the 



