77 



mountains to their present height may have been a gradual process, 

 during the continuance of which, in waters constantly becoming 

 shallower, the strata of slate may have been exposed to further 

 extensive denudation, which, joined to various atmospheric influences, 

 afterwards acting, would give their present form and outline to 

 the jagged ridges of the northern mountains. Long before this 

 elevation took place, the granite, under the pressure of the superin- 

 cumbent slate, and perhaps of the newer formations also, had acquired 

 its crystalline structure by the slow passage of its heat of fusion into 

 the adjoining strata ; and most probably it was quite solidified ante- 

 riorly to its elevation, so that it was protruded in a solid form. 



53. The agent in this protrusion may have been a newer granite, 

 produced beneath the former. Let fresh accessions of molten matter 

 the matter of granite be slowly and constantly transfused from 

 the nether depths, amid the basement portion of the older granite, 

 already cooled and crystalline above, while fused below by contact 

 with the molten mass, this latter will expand, and perhaps later- 

 ally extend the former, and raise it in a solid form. Thus a great 

 upward movement might be produced, forming the high mountain 

 nucleus of the north, and at the same time elevating and contorting 

 the strata of slate and sandstone resting on the flanks of the older 

 granite, and in some places perhaps even inverting the dip of the 

 slate, as being subject to a greater strain, and more likely to yield en 

 masse, without disruption ; while the sandstones of the southern pla- 

 teau, remote from the focus of intensity in the upheaving force, 

 would be elevated from below in more horizontal strata. The newer 

 granite below might likewise impenetrate the older, and so make its 

 appearance in lower situations, when the land was finally raised, and 

 had assumed, in virtue of denudations effected during the process, 

 and by causes afterwards acting, somewhat of the aspect which it 

 now retains. Formed under such conditions, this later granite 

 might be expected to differ in structure, if not also in composition, 

 from the older granite, invaded and displaced by it. Such differ- 

 ences we know do actually exist between granites in the Alps, Andes, 

 and other localities, which can be clearly proved to be of different 

 ages. Now, we have two such granites within the area of the moun- 

 tain nucleus the coarse and the fine-grained whose limits have 

 been already described, and whose mineralogical differences, as ori- 

 ginally recognized by MacCulloch, have been pointed out (Art. 46). 

 It is with this latter, or fine grained variety, deficient in mica, 

 that Mr. Ramsay's Ploverfield and our Craig-Dhu granite almost 

 exactly agree (Art. 50, 51). Is it not, then, probable that these 



