TBANSACTIONS OF SECTION C. 739 



servers in this country had to prepare their own sections, and those who, like 

 myself, have had the privilege of examining many of his slides scarcely know 

 which to admire most — the skill and patience of which they are the evidence, or 

 the conciseness and accuracy of his petrographical descriptions. His papers do 

 not occupy a large number of pages, but they are based on an amount of obser- 

 vation which is truly surprising. The general conclusions at which he arrived as 

 to the essential identity of ancient and modern igneous rocks are expressed with 

 the utmost confidence, and one feels, after going over his material, that this con- 

 fidence was thoroughly justified. It is curious now to note that the one British 

 champion of the distinctness of the Tertiary and pre-Tertiary rocks pointed to the 

 difference between the Antrim and Limerick traps. These traps differ in exactly 

 the same way as do the corresponding Tertiary and pre-Tertiary continental rocks, 

 with this important difference. On the Continent the ophitic structure is cha- 

 racteristic of the pre-Tertiary rocks, whereas in the north of Ireland it is a 

 marked feature of those of Tertiary age. We see, therefore, that the arguments 

 for the distinctness of the two sets of rocks derived from the two areas, based in 

 both cases on perfectly accurate observations, neutralise each other, and the case 

 hopelessly breaks down as regards the basalts and dolerites. 



In this country it is now generally recognised that, when allowance is made 

 for alterations which are necessarily more marked in the earlier than in the later 

 rocks, there is no important difference either in structure or composition between 

 the rhyolites, andesites, and basalts of the Palfeozoic and Tertiary periods. But 

 identity of structure and composition may in this case be taken to imply identity 

 as to the physical conditions under which the rocks were produced. We are thus 

 led to picture in our minds long lines of volcanoes fringing the borders of Palaeo- 

 zoic continents and rising as islands in the Palteozoic seas. Then, as now, there 

 issued from the craters of these volcanoes enormous masses of fragmental material, 

 a large portion of which was blown to dust by the explosive escape of steam and 

 other gases from the midst of molten rock ; and then, as now, there issued from 

 fissures on their flanks vast masses of lava which consolidated as rhyolite, andesite, 

 and basalt. We may sum up the case as regards the volcanic rocks by saying that, 

 80 long as observations are confined to a limited area, doubts may arise as to the 

 truth of the uniformitarian view, but these doubts gradually fade away as the area 

 of observation is extended. There are still some outstanding difficulties, such as the 

 apparent absence of leucite lavas amongst the Palaeozoic formations; but as many 

 similar difficulties have been overcome in the past, it is improbable that those 

 which remain are of a very formidable character. 



So far we have been referring to rocks formed at the surface of the earth 

 under conditions similar to those now in operation. But there are others, such as 

 granite, gneiss, and mica-schist, which are obviously unlike any of the products of 

 surface agencies. If these rocks are forming now, it must be beneath the surface. 

 This point was clearly realised by Hutton. Granite was proved by him to be an 

 igneous rock of subterranean origin. His conclusions as to the formation of the 

 schists are expressed in a passage so remarkable when viewed in connection with 

 what I regard as the tendency of modern research that I make no apology for 

 quoting it at length. ' If, in examining our land, we shall find a mass of matter 

 which had been evidently formed originally in the ordinary manner of stratifica- 

 tion, but which is now extremely distorted in its structure, and displaced in its 

 position — which is also extremely consolidated in its mass, and variously changed 

 in its composition — which therefore has the marks of its original or marine com- 

 position extremely obliterated, and many subsequent veins of melted mineral 

 matter interjected ; we should then have reason to suppose that here were masses 

 of matter which, though not different in their origin from those that are gradually 

 deposited at the bottom of the ocean, have been more acted upon by subterranean 

 heat and the expanding power, that is to say, have been changed in a greater 

 degree by the operations of the mineral region. If this conclusion shall be 

 thought reasonable, then here is an explanation of all the peculiar appearances 

 of the Alpine schist us masses of our land, those parts which have been erroneously 



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