DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 45 



4. Petrographical Nomenclature. — It is obvious that no one term will suffice to 

 describe a series of rocks presenting such, variety of mineralogical and chemical composi- 

 tion as those that form the system of dykes now under discussion. Basalt, dolerite, 

 melaphyre, diabase, augite-porphyrite, diabase-porphyrite, augite-andesite, and doubtless 

 other names might be appropriately applied to different dykes ; sometimes, indeed, 

 more than one of these terms might be given to different parts of the same dyke. When 

 the geological history of the dykes, and their connection with the rest of the volcanic 

 phenomena are the subject of inquiry, rather than their petrographical characters, it 

 becomes convenient to adopt some general term which may appropriately describe the 

 whole. The word " basalt " has been so employed, but it is obviously so inapplicable to 

 the more acid and andesitic rocks that its use as a general epithet is objectionable. None 

 of the other specific names are free from the same defect. The old term " trap " would be 

 useful, but it has become obsolete, and its revival might be attended with grave dis- 

 advantages. In referring to the dykes of this great system, therefore, I shall speak of 

 them as "the basic dykes," reserving specific names for such individual cases as may 

 require them. 



§ 4. Hade. 



In the great majority of cases the dykes are nearly or quite vertical. This position 

 is more particularly exhibited by the great single dykes. But occasionally, where one 

 of these crosses a deep valley, a slight hade is perceptible by the deviation of the 

 line of the dyke from its normal course. Sedgwick long ago noticed that the 

 Cleveland dyke has, in places, an inclination of at least 80° to its N.E. side.* In 

 the coal-workings, also, a trifling inclination is sometimes perceptible, especially where a 

 dyke has found its way along a previously existing line of fault, as in several examples 

 in Stirlingshire. But in those districts where the dykes are gregarious, departures from 

 the vertical position are not infrequent, more particularly near the great basalt-plateaux. 

 It was long ago noticed by Necker, that even in such a dyke-filled region as Arran, almost 

 all of the dykes are vertical, though sometimes deviating from that position to the extent 

 of 20°.t Berger found that the angle of deviation among those of the north of Ireland 

 ranges from 9° to 20°, with a mean of 13°. J The most oblique examples are probably 

 those which occur in the basalt-plateaux of the Inner Hebrides, where the same dyke in 

 some parts of its course runs horizontally between two beds, across which it also ascends 

 vertically (see fig. 41). But with these minor exceptions, the verticality of the great 

 system of dykes, pointing to the perpendicular fissure-walls between which the molten 

 rock ascended, is one of the most notable features in their geological structure. 



§ 5. Breadth. 



An obvious characteristic of most dykes is the apparent uniformity of their breadth, 

 Many of them, as exposed along shore-sections, vary as little in dimensions as well- 



* Cambridge Phil. Trans., ii. p. 28. t Trans. Roy. Soc. Edin., xiv. p. 677. t Trans. Geol. Soc, iii. p. 227. 



VOL. XXXV. PART 2. G 



