528 report — 1878. 



that we have here the result of an ordinary eruption, where lava is ejected, and we 

 are forced to believe that it was altogether aeriform. From the great extent of 

 country over which the agglomerate occurs, it is not improbable that the explosion 

 which accompanied the protrusion of the nearly horizontal dyke affected a con- 

 siderable area at the same time, disrupting and triturating to powder the overlying 

 rocks, the vast volume of which, falling back into the opening, together with the 

 rapid consolidation of the igneous mass consequent upon so great a disengagement 

 of the interstitial vapotir, combined to check the volcanic activity before it could 

 produce scoriae or lava. 



Another point of interest in connection with this district is the evidence it 

 affords of the connection between the plutonic and the volcanic rocks. This is not 

 indeed as fully exemplified here as in other districts, as, for instance, in the Western 

 Isles of Scotland, where Professor Judd traces all the gradations from granite to 

 pumice and scoria, yet it is sufficiently illustrative of the principle when we find 

 plutonic rocks graduating into others which by their protrusion have produced me- 

 chanical accompaniments. 



3 Notes on the Glaciation of Ireland, and the Tradition of Lough Lurgan. 

 By W. Mattieu Williams, F.R.A.S., F.O.S.* 



In comparing the vestiges of ancient glaciation (especially those of Norway 

 and Ireland) with the deposits of existing Alpine glaciers, the author has been 

 much impressed with the very general fact that, although the " till " and other 

 varieties of boulder clay are of unquestionable glacial origin, they are unrepresented 

 by the moraines, &c, of the glaciers now existing in the temperate zones ; and on 

 the other hand true representatives of recent Alpine moraines are very rarely 

 found among the ancient glacial deposits. 



An explanation of this is offered. Living Alpine glaciers (with a few ex- 

 ceptions) terminate on mountain slopes, and are sources of mountain torrents. 

 These torrents wash out, carry away, and afterwards deposit as a distinct alluvium 

 the material corresponding to the clayey matrix of the ancient drift, leaving 

 behind only the boulders and smaller stony particles to form the stony heaps of 

 modern moraines. 



The remarkable absence of such moraines on the very long and remarkably 

 "•lariated coast line and mountain slopes of Scandinavia is explained by the fact 

 that the glaciers of that region originated in a great neve, covering the table-land 

 fields which overflowed down their boundary valleys into the sea. 



The great central plain of Ireland is compared to such a fjeld. During the 

 glacial epoch it was covered with ice, was a great neve or glacier source, and its 

 outlet glaciers flowed on all sides over the surrounding mountain barriers down 

 their seaward slopes into the ocean and terminated there, leaving the bulk of their 

 mineral burdens in the sea. 



Glaciers thus outthrust upon the waters under a climate in which the annual 

 snowfall exceeded the annual thaw, would be thinned from below and grow from 

 above, and thus extrude their mineral matter downwards instead of upwards, as in 

 Alpine temperate-zone glaciers. 



They would present a further inversion of such glaciers, by bending upwards 

 by flotation as they advanced, instead of downwards by gravitation ; and their cre- 

 vasses would therefore be formed at the lower instead of the upper surface of the 

 ice, and would gape downwards instead of upwards. 



As they became thawed below by the friction of their advance they would 

 accumulate a vast amount of debris at their base, effecting far more erosion than 

 the clearer ice-bottom of modern Alpine glaciers. Thus, more fine slimy im- 

 palpable powder or clay material would be formed, and this, in its slimy condition . 



* The views of the constitution of ancient glaciers, and the formation of boulder 

 clay, contained in this communication, are more fully discussed in a paper on ' The 

 Great Ice Age and the Origin of the Till,' published in the ' Quarterly Journal of 

 Science ' of April, 1877. 



