324 UK. ALFRED HARKER ON THE GABBRO [Aug. 1 894, 



One point brought out by these analyses is that the augite, and 

 the uralitic hornblende derived from it, must be rich in alumina. 

 Another point of interest is that the iron ore is in a high degree 

 titaniferous. If from the second analysis we calculate the iron ores 

 as magnetite and ilmenite, the percentages of these are found to be 

 12-24 and 9-93 respectively, the two together thus constituting 

 22*17 per cent, of the whole rock. 



Looking simply at the bulk-analyses, we observe that while 

 analysis II. shows more than four times as much iron oxides as I., 

 and nearly four times as much magnesia, it shows about twelve 

 times as much titanic acid. In other words, regarding the second 

 rock as a basic modification of the first, the titanic acid is much 

 more strongly concentrated in the basic fades than the iron-oxides 

 are. This is in agreement with what is recorded in other districts, 

 such as those of Ekersund and Taberg. Vogt 1 remarks as character- 

 istic of this type of modification ('oxidic' secretion of iron ores) that 

 there is never less titanic acid than that corresponding to the rela- 

 tion Ti : Fe = 1 : 10, and often considerably more. In our rock 

 the ratio is 1 : 5*3. As regards the other constituents, which were 

 roughly estimated for the second rock, though exact figures are not 

 given, analysis II., as compared with I., shows, in addition to the 

 great falling off in silica, a very considerable reduction in lime, and 

 a certain diminution in the percentage of soda, while the potash 

 disappears almost entirely. Phosphoric acid has not been estimated, 

 but the microscope shows that apatite, which is scarcely to be 

 found in most specimens of the more acid varieties of the gabbro, 

 becomes locally abundant in the highly basic marginal rocks. In 

 all these particulars, the variation observed here resembles that 

 recorded by Vogt and others in other parts of the world. 



The Carrock Fell gabbro illustrates, then, a clearly characterized 

 type of continuous variation in a single intrusive mass of basic 

 rock, the variation being related in a simple manner to the boundary 

 of the mass. It cannot be doubted that all the varieties have been 

 derived by the differentiation of a single magma after its intrusion, 

 and that such differentiation consisted in a concentration of what 

 we may call the more basic constituents of the magma in the 

 marginal parts. The phenomena thus afford an opportunity of 

 bringing to the test some of the ideas that have been put forward 

 with reference to the probable causes of differentiation in rock- 

 magmas, and this I shall briefly attempt to do. 



5. Discussion of the Causes of such Variation. 



Several possible causes of differentiation have been suggested, 

 and one or other of them may have been the chief cause in particular 

 instances. In the present case the circumstances enable us to 

 eliminate at once some of these suggestions. It may be remarked 

 first that the concentration of the basic constituents is found along 

 both the northern and the southern margin of the mass. These 

 1 Zeitschr. fur prakt. GeoL vol. i. (1893) p. 10. 



