30 DR GEIK1E ON THE HISTORY OF VOLCANIC ACTION 



observations of Winch that these dykes, though they ascend through the Coal Measures, 

 never enter the Magnesian Limestone. They differ also in direction from the younger 

 dykes, their general trend being south-westerly. They are further distinguished by petro- 

 graphical characters. 



But when these and all other dykes which can reasonably be referred to older 

 geological periods are excluded, there remains a large majority which cannot be so 

 referred, but which are connected together by various kinds of evidence into one great 

 system that must be of late geological date, and can be assigned to no other than the 

 Tertiary period in the volcanic history of Britain. In my original memoir, "On the 

 Chronology of the Trap-rocks of Scotland," where I first drew attention to this great 

 system of dykes in connection with the progress of volcanic action in the country, I 

 pointed out the grounds on which it seemed to me that these rocks belonged to a com- 

 paratively late geological date. My own subsequent experience and the full details of 

 structure collected by my colleagues of the Geological Survey in all parts of the country, 

 have amply confirmed this view, though, as already stated, instead of placing the era of 

 eruption in the Jurassic, I now put it in the older part of the Tertiary period. The 

 characters which link this great series of dykes together as one connected system of late 

 geological date are briefly enumerated in the following list, and will be more fully discussed 

 in later pages. 



1. The prevalent tendency of the dykes to take a north-westerly or westerly course. 

 There are exceptions to this normal trend, especially where the dykes are small and 

 locally numerous ; but it remains singularly characteristic over the whole region. 



2. The increasing abundance of the dykes as they are traced to the west coast and 

 the line of the great Tertiary volcanic plateaux of Antrim and the Inner Hebrides. 



3. The rectilinear direction so characteristic of them and so different from the tortuous 

 course of other groups of dykes. The exceptions to this normal feature are as a rule 

 confined to the same localities where departures from the prevalent westerly trend occur. 



4. The great breadth of the larger dykes of the system and their persistence for long 

 distances. This is one of their most remarkable and distinctive characters. 



5. The posteriority of the dykes to the rest of the geological structure of the regions 

 which they traverse. They are not only younger than the other rocks, but younger 

 than nearly all the folds and faults by which the rocks are affected. 



6. The manner in which they cut the Jurassic, Cretaceous, and older Tertiary strata 

 in the districts through which they run. At the south-eastern end of the region they 

 rise through the Lias and Oolite formations, in the west they intersect the Chalk and 

 the Tertiary volcanic plateaux with their later eruptive bosses. 



7. Their petrographical characters, among which perhaps the most distinctive is the 

 frequent appearance of the original glassy magma of the plagioclase-pyroxene- 

 magnetite (olivine) rock, of which they essentially consist. This glass, or its more or less 

 completely de vitrified representative, often still recognisable with the microscope among 

 the individualised microlites and crystals throughout the body of a dyke, is also not 



