MR MILNE ON THE GEOLOGY OF ROXBURGHSHIRE. 45'() 



elevated and squeezed into their present condition by forces, which acted over a 

 considerable portion of the earth's crust. 



Whether they acted every where contemporaneously, so that the greywacke 

 hills, in the south of Scotland, were raised at the same instant, as those of Cum- 

 berland and Perthshire, may be doubtful. For there is no reason why the same 

 force might not act at different places, at several successive periods. But there 

 is certainly strong reason for thinking, that it was the same force which acted on 

 the Grampians, the Lammermuirs, and the Cumbrian chains ; (1.) Because they 

 consist of rocks, of apparently the same age, having been all deposited before the 

 epoch of the old red sandstone formation ; (2.) Because the effects on all these 

 greywacke chains of hills are precisely similar. 



What that gigantic force was, or could have been, which produced effects so 

 remarkable for their extent and their uniformity, is a question too difficult and 

 too general to be entered on here. One theory is, that these greywacke rocks 

 were raised by the effects of subterranean heat. Another theory is, that the in- 

 ternal nucleus of the globe is, from excess of heat, in a molten and liquefied state ; 

 and that the temperature of this nucleus diminishes faster than the crust, so that, 

 as the nucleus contracts in size, or even changes in form, the external crust, in 

 order to accommodate itself to what it rests on, must be broken up, and occupy 

 smaller space than before. 



Without entering upon the discussion of these two theories, I may observe, 

 that if, as the former implies, the greywacke strata were upturned by volcanic 

 action, it is reasonable to suppose that there would have been large outbursts of 

 volcanic rocks among these strata. But, so far from this being the case, the grey- 

 wacke formation in the south of Scotland, and especially in Roxburghshire, is, 

 generally speaking, entirely exempt from igneous rocks ; and where igneous rocks 

 do exist, as in Ayrshire and among the Lammermuir Hills, the adjoining grey- 

 wacke formation has lost many of its ordinary characters, as, for instance, the 

 east and west strike of its beds. Indeed, it is not easy to imagine how igneous or 

 volcanic action could have operated, so as to produce the remarkable parallelism 

 of chains and strata which distinguishes this ancient formation. The outbursts 

 of igneous rocks seldom or never form continuous chains, at least of any extent ; 

 but, according to the theory now alluded to, one would have expected to have 

 found a central axis of igneous rocks stretching across the island, having on each 

 side the greywacke strata which it had been the means of raising up. 



On the other hand, if, from the refrigeration and contraction of the earth's 

 nucleus, its crust became rent and broken, it is easy to see how, through these 

 rents, portions of the molten nucleus might have squirted up, and formed those hills 

 of granite and other ancient igneous rocks which occasionally occur in, and on 

 the outskirts of, the greywacke formation. The effects, therefore, which such a 



VOL. XV. PART III. 6 L 



