Vol. 50.] OF CAEROCK FELL. 323 



mass. la other words, there has been, from whatever cause, a 

 concentration of the iron oxides and certain other constituents in 

 the marginal portion of the mass. 



Phenomena indicating a concentration of this kind have been 

 recorded in numerous instances from different parts of the world. 

 Prof. Yogt ] has recently reviewed the literature of the subject 

 and critically examined all the leading facts ; so that it is not 

 necessary here to enter upon so wide a field. The phenomena are 

 characteristically found in basic and ultrabasic rocks, and in extreme 

 cases have given rise to almost pure aggregates of iron ore of un- 

 doubtedly eruptive origin. Vogt distinguishes especially an ' oxidic ' 

 type of concentration, characterized by the secretion of titaniferous 

 iron-oxides, and a ' sulphidic ' type, characterized by nickeliferous 

 iron- sulphides. At Carrock Fell we evidently have to do with the 

 former type. According to Vogt the titaniferous iron-oxides tend 

 to aggregate by preference in the central part of an eruptive mass, 

 while the nickeliferous iron-sulphides concentrate in the marginal 

 part. As regards the former, this generalization seems to go rather 

 beyond the facts, and the case that I am describing is emphatically 

 opposed to it. In some of the cases which Vogt notices of aggre- 

 gates of iron ores in the heart of an eruptive mass, the aggregates 

 evidently are abruptly bounded, and their secretion from the magma 

 must have taken place before the intrusion, so that the original 

 relations are lost. Where a perfectly graduated transition indicates 

 a differentiation of the intruded magma in situ, the enrichment in 

 iron oxides, etc., seems to be typically a marginal phenomenon. 



I now proceed to examine more closely the variations in the 

 chemical composition of the gabbro. My friend, Mr. G. Barrow, of 

 the Geological Survey of Scotland, has had the kindness to make a 

 complete analysis of one selected specimen and a partial analysis of 

 another. The former is an example of the quartz-bearing gabbro, 

 though not actually the most acid rock found. The latter is the 

 densest and most basic specimen obtained, having an extraordinarily 

 large proportion of iron ores. 



I. II. 



SiO, 5350 3253 



TiO" 2 0-45 5-30 



A1,0 3 22-20 



Fe 2 3 3-60 844 



FeO 2-64 1710 



MnO 035 



MgO 200 7-92 



CaO 945 



Na„0 4-26 



K 2 0-61 



Ignition l - 50 



10059 



Specific gravity ... 2-800 3-265 



I. Quartz-gabbro, by roadside, 150 yards N.N.W. of Chapel Stone. (The 

 lime is probably a little too bigb.) 

 II. Iron-ore gabbro, upper part of Furthergill Sike. 



1 Zeitschr. fur prakt. Geol. vol. i. (1893) pp. 4-11, 125-143, 257-284. 



