MR MILNE ON THE GEOLOGY OF ROXBURGHSHIRE. 477 



mechanical deposition of different kinds of sediment. I cannot help thinking 

 that substances of the same nature, and having chemical affinities, have after- 

 wards arranged themselves into beds. I have, however, no evidence to adduce 

 in support of this view, and I offer it as little better than a conjecture to account 

 for a problem in geology, which, it appears to me, has not yet been solved. But 

 it has always appeared to me that geologists have taken too little into account 

 the important effects which might result from chemical action continuing for a 

 long period to modify the arrangement and character of the sedimentary strata. 



4. After the deposition of the red sandstone and carboniferous rocks, another 

 outburst of igneous rocks took place, of all kinds. The amygdaloid and breccia 

 of the Eildon Hills, of the Minto Hills, of Bedrule, of Ancrum Crag, and Wooden 

 Burn, then flowed up, as well as those great coulees of porphyry already refer- 

 red to. 



Many, indeed most, of these newer volcanic rocks are in contact with, or in 

 the immediate neighbourhood of, igneous rocks of a much older date. But this is 

 just what might have been expected, as those parts of the earth's crust, once 

 burst through, would continue to be weak points, and afford less resistance than 

 others to the expulsion of volcanic matter. 



Besides those eruptions of trap, which now form hills and coulees, there be- 

 long to the epoch now referred to, the greenstone and basaltic dykes. It is a curi- 

 ous circumstance that these dykes all run very nearly parallel to one another, 

 viz. about west-north-west by compass ; and that this also is the direction of all 

 the principal dykes in Northumberland and Durham. Further, it is deserving of 

 remark, that the Hawick dyke, which I have traced for above 26 miles continu- 

 ously, and at its south end crosses the Cheviot range of hills, appears to coincide 

 with one or other of the basaltic dykes running into the sea on the Northumber- 

 land coast. Mr Wood,* in his account of the rocks on the shore between Berwick 

 and Newcastle, speaks of a basaltic dyke near Howick running N. 58 W.,f and 

 which, whilst it agrees in direction with the Hawick dv-ke, seems to cut through 

 the Cheviots not far from the place where the Hawick dyke runs. If the conclu- 

 sion to which these circumstances point, be verified by a farther examination of 

 the course of this dyke, it will then be found to stretch in one unbroken line for 

 at least 50 miles, and without at either end shewing any signs of cessation.:): 



* Transactions of Newcastle Natural History Society, vol. i. p. 308. 



t Mr WOOD'S statement is N. 83 W., which, it is presumed, are true bearings. 



J Mr ADAM MATHESON, millwright, Jedburgh, already referred to for his geological zeal, has lately 

 afforded additional proof of this, by actually attempting to trace the dyke from the Scottish Border through 

 Northumberland to the sea. Having intimated to me his intention of setting out on this voyage of dis- 

 covery, and asked me for instructions, I sent him out a map, compass, and other necessary implements. 

 He writes me, that he hired a horse at Jedburgh, and set out from Hindhope along the line which, at that 



VOL. XV. PART III. 6 N 



