GYMNOSPERM.E. 77 



determination of the ages of strata, and their importance has thus come to be regarded 

 in this country as trivial or at least subordinate. The fact, more and more apparent 

 every day in other countries, and now brought home to us in Ireland and in Scotland, 

 that the determination of the ages of strata of vast area and thickness sometimes rests 

 entirely on the plants imbedded in them, must eventually, however, open our eyes to their 

 importance, and those who contribute in any way to the advancement of their study will 

 have the satisfaction of contrasting our present knowledge and estimation of them with 

 what we may anticipate it to become when this work has reached its conclusion. 



The Basaltic Formation of Ireland and Scotland. 



The Basalts of Ireland have frequently formed the subject of communications to 

 local and other scientific bodies. Their authors have, however, been uniformly satisfied 

 to accept the current opinions as to their age, based upon the scanty and imperfectly 

 understood plants found in them ; and none of the descriptions are of an entirely com- 

 prehensive or exhaustive character. The Scottish Basalts have been more broadly treated 

 of by Professors Geikie and Judd ; but, as with those of Ireland, no clue to their precise 

 age exists, except their plant remains. The only locality hitherto known to yield them in 

 a satisfactory condition is Ardtun Head, in Mull ; but a locality in the Isle of Canna has 

 been discovered which may supplement the former Flora. The leaf-bed here forms the 

 roof of a small cave, and the specimens obtained have become very friable from exposure 

 to weather. I think it will be better to defer the description of the Scotch Basalts until 

 I have been able to visit personally and collect from the new locality. 



The Basalts of Ireland extend over an area that is roughly estimated at 1200 square 

 miles, comprising nearly the whole of Antrim and portions of the adjoining counties of 

 Londonderry and Tyrone. They form an elevated plateau bounded for the most part by 

 noble escarpments with precipitous flanks, rising to elevations of 1000 and 1500 feet. 

 Their total thickness is but 1100 feet, though these igneous rocks are estimated to 

 reach in Mull not less than 3000 or 4000 feet. 



These Basalts are but a small fragment, the mere southern limit, of a series of 

 stupendous outpours of lava that once extended probably in an unbroken mass north- 

 ward through Scotland and the Faroes to Iceland, where they are of unknown thickness. 

 Nor would this represent their extreme limits, as may be seen in Scotland, where 

 innumerable dykes diverge from them and penetrate through England to the North Sea. 

 Their northern extension is as yet unknown, and their boundaries may probably have to 

 be traced in Greenland and even Spitzbergen. Vast as this area is, it is not wholly 

 without parallel elsewhere. A region in the Deccan, 200,000 square miles in extent, has 

 been converted into a plateau of horizontal sheets of Basalt whose aggregate thickness is 

 6000 feet. These Indian flows are of Cretaceous age, and therefore preceded ours ; but in 



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