75 



(led in thu stratii of the coal-measures, and utiier similar phenomena, 

 have led the author to suppose, that these rivers and their estuaries 

 may have been of greater magnitude than would probably have been 

 the case if they had been situated in small islands, according to the 

 opinion of many geologists. The intermixture of terrestrial and ma- 

 rine remains in the same beds, is a strong evidence in favour of their 

 fluviatile origin ; and the fact frequently observed, of these beds 

 being covered by, or alternating with, others containing only marine 

 remains, may, with great probability, be referred to changes in the 

 relative level of the land and sea that may have taken place while 

 these deposits were forming. 



In the author's opinion, it is still doubtful whether any beds have 

 yet been discovered in this series which may be considered to be ex- 

 clusively of fresh-water origin, unless an exception should be found 

 in the limestone noticed by Mr Murchison at Pouterburg in Shrop- 

 shire. jM. Affassiz has shown, that neither the Burdiehouse lime- 

 stone, nor any of the other beds of the Scottish coal-fields with which 

 we are at present acquainted, is of that character ; nevertheless the 

 limestone at Burdiehouse is a very remarkable deposit; and the dis- 

 coveries of Dr Hibbert with reference to that locality, are of the 

 highest geological importance. 



The author then proceeded to describe the limits within which the 

 coal appears to have been deposited in the Scottish Lowlands, which, 

 with the exceptions pointed out by him, may, according to Williams, 

 be indicated by a line drawn from the mouth of the Tay, passing 

 through Stirling, to the northern extremity of Arran; and another 

 nearly parallel to it from St Abb's Head on the east coast to Girvan 

 on the west. Although coal may not have been equally distributed 

 in every part of this district, — the deposition of the vegetable matter 

 from which It was derived, having probably been more or less in- 

 fluenced by local circumstances, which may also have caused occa- 

 sional varieties in the mineral structure and organic contents of the 

 associated strata, — yet, in the opinion of the author, there are suffi- 

 cient grounds to justify the conclusion, that the whole series origi- 

 nally constituted one great formation, the strata of which it is com- 

 posed appearing to have been deposited continuously, more or less, in 

 a horizontal position at the bottom of the sea, that must then have 

 covered at least the whole of that portion of the Lowlands, forming 

 either a strait or channel between two islands, or perhaps a vast es- 

 tuary in which the rivers of the neighbouring primeval countries dis- 

 charged their waters. The ripple-marks observable on the surface 



