Not less remarkable than their length is the preservation of their normal petrographical characters by some dykes for long distances. In this respect the Cleveland dyke may again be cited as a typical example. The megascopic and microscopic structures of the rock of this dyke distinguish it among the other eruptive rocks of the North of England. And these peculiarities it maintains throughout its course.[184] Similar though less prominent uniformity may be traced among the long solitary dykes of the South of Scotland, the chief variations in these arising from the greater or less extent to which the original glassy magma has been retained. The same dyke will at one part of its course show abundant glassy matter even to the naked eye, while at a short distance the vitreous groundmass has been devitrified, and its former presence can only be detected with the aid of the microscope. Where a dyke has caught up and absorbed abundant foreign materials its composition naturally varies considerably from point to point. Mr. Harker has observed some good examples of this variation in Skye. [Footnote 184: See the careful examination of this dyke by Mr. Teall, _Quart. Journ. Geol. Soc._ xl. p. 209.] CHAPTER XXXV THE SYSTEM OF DYKES--_continued_ Direction--Termination upward--Known vertical Extension--Evidence as to the movement of the Molten Rock in the Fissures--Branches and Veins--Connection of Dykes with Intrusive Sheets--Intersection of Dykes--Dykes of more than one infilling--Contact Metamorphism of the Dykes--Relation of the Dykes to the Geological Structure of the Districts which they traverse--Data for estimating the Geological Age of the Dykes--Origin and History of the Dykes. 9. DIRECTION Another characteristic feature of the dykes is their generally rectilinear course. So true are the solitary dykes to their normal trend that, in spite of varying inequalities of surface and wide diversities of geological structure in the districts which they traverse, they run over hill and dale almost with the straightness of lines of Roman road. In the districts where the gregarious type prevails, the dykes depart most widely from the character of the great solitary series, but still tend to run in straight or approximately straight lines, or, if wavy in their course, to preserve a general parallelism of direction.