320 ME. ALFRED HARKER ON THE GABBRO [Aug. 1 894, 



indicate lines of flow. This structure seems to be quite independent 

 of the more general variation in the gabbro-mass as a whole, which 

 I shall describe below. At several localities the banding was 

 observed to dip steeply to N.N.E., which agrees with what seems 

 from other evidence to be the lie of the intrusion as a whole. Near 

 Round Knott, however, there are lower and undulating dips, often 

 southerly. In most parts of the gabbro no banding is observable. 



The finer-textured portions of the gabbro have in the field a 

 generally darker look than the coarser parts, which seem more 

 felspathic. This — a common observation in such rocks — is perhaps 

 in part illusory. There are, however, considerable local variations 

 in the mineralogical composition of the gabbro, which are possibly 

 connected with the variations in texture. Thus, at a few spots the 

 rock is very rich in augite, the lustrous surfaces of that mineral 

 appearing in a hand-specimen to make up by far the greater part of 

 the whole. At no great distance this variety may be found to give 

 place to one in which felspar and magnetite are richly represented. 

 Gabbros are well known to be peculiarly liable to such variability, 

 which does not necessarily import any very great difference in 

 chemical composition between the several varieties. The felspar 

 and augite probably do not differ much in silica-percentage, while 

 the iron which goes into the pyroxene in one case goes into the 

 magnetite in the other, the chief differences being probably in the 

 alumina and soda. However this may be, the essential differences 

 which these irregular mineralogical variations denote are certainly 

 far less than those to be discussed next, which have a definite 

 arrangement and an important significance. An account of these 

 wider and more general variations will show conclusively that the 

 gabbro represents a single intrusion of igneous magma, which was 

 all thoroughly fluid at one time ; so that the abrupt local changes 

 and banded structure cannot be explained as the results of successive 

 interlacing injections. 



4. Orderly Variation from Centre to Margin. 



Apart, then, from minor local variations, we must remark that 

 specimens of the gabbro from different localities show wide 

 differences both chemically and in the relative proportions of their 

 minerals ; and a study of the rocks in the field soon shows that the 

 more acid varieties occur in the central part of the mass, the more 

 basic near the edge. Two chemical analyses from different localities 

 are given below : besides these I have had several silica-percentages 

 kindly determined for me by Messrs. W. A. Brend and E. H. Cun- 

 ningham-Craig. 1 The highest silica-percentage is 59*46 for a 

 rock taken near White Crags ; the lowest is 32-50 for the northern 

 margin of the mass as exposed in the upper part of Furthergill Sike. 

 The other figures obtained accord very fairly with the localities of 



1 The determinations were made by Messrs. Brend and Cunningham-Craig 

 in the laboratory of Sidney Sussex College, Cambridge. Since writing the above, 

 I have received the results of others made by Messrs. Boyd, Fry, Gunnell, 

 Gutbrie, King, and Peatfield, at the Yorkshire College, Leeds : these I owe to 

 the kindness of Dr. J. B. Cohen. The two sets of figures are distinguished 



