Vol. 50.] BANDED STRUCTURE OF SOME TERTIARY GABBROS. 655 



over the augite, and assumes in some cases a more compact character 

 than is the case in the other rocks. Taking the rocks of Druim an 

 Eidhne as a whole, the compact hornblende which forms so marked 

 a feature in the foliated gabbros of the Lizard is conspicuous by its 

 absence, and so also is the saussuritic modification of the felspar. 

 The specific gravities of two specimens from the veins were found 

 to be 2- 78 and 2*85 respectively. 



III. General Deductions. 



From the facts which have been here described we may proceed 

 to indicate some of the conclusions which seem to us to be legiti- 

 mately deducible from them. There are two lines of enquiry on 

 which these facts may be made to throw some light. In the first 

 place we may enquire how far they serve to extend our knowledge 

 of the conditions under which igneous magmas may be protruded and 

 consolidated, and in the second place we may consider to what 

 extent the phenomena exhibited by these Tertiary gabbros serve to 

 elucidate the structure and origin of the oldest gneisses. 



(i) We are accustomed to think of an igneous magma as fairly 

 uniform in its composition at the time of its intrusion into 

 surrounding rocks or extrusion at the surface ; and in the majority 

 of cases this view is probably correct, so far as the massive igneous 

 rocks are concerned. If samples equal in bulk to that of an average 

 hand-specimen could be taken from various portions of the molten 

 mass at the time of its rise to the place where the igneous rock is 

 now observed, they would probably be found to possess the same, 

 or very nearly the same, chemical composition. Crystals are no 

 doubt often present in the magma at the time of its intrusion or 

 extrusion, but they are usually distributed with approximate 

 uniformity through the still molten material, and do not, therefore, 

 affect the question we are now considering. As an illustration of 

 uniformity in the composition of one and the same mass of rock we 

 may refer to the Cleveland Dyke, which has been traced at intervals 

 for a distance of 90 miles across the North of England. It traverses 

 Jurassic, Triassic, and Carboniferous strata of the most diverse petro- 

 graphical characters without undergoing any marked change either 

 in composition or structure. 1 



Turning now to larger masses of rock which occur as sills, lacco- 

 lites, and bosses, we find that uniformity in composition is not so 

 marked a feature as it is in the majority of dykes. 2 Such differences 

 as occur may, however, in many cases be explained by differentiation 

 subsequent to intrusion — differentiation in situ as it may be termed — 



1 ' Penological Notes on some North-of-England Dykes,' by J. J. H. Teall, 

 Quart. Journ. Geol. Soc. vol. xl. (1884) p. 209. 



2 But dykes are not always uniform in composition. See A. Geikie, ' The 

 Pitchstone of Eskdale,' Proc. Roy. Pbys. Soc. Edin. vol. v. (1880) p. 219 ; 

 A. C. Lawson, ' Petrographical Differentiation of certain Dykes of the Rainy 

 Lake Region,' 'American Geologist,' vol. vii. (1891) p. 153; Prof. J. W. Judd, 

 ' On Composite Dykes in Arran,' Quart. Journ. G-eol. Soc. vol. xlix. (1893) p. 536. 



