of the Island of Bute. 83 



of Perthshire. The valleys intersecting the island seem ob- 

 viously a part of that great system of parallel fractures which 

 run in a north-east and south-west direction on both sides of 

 the Grampians, and are probably due to the original upheaval 

 of that chain, and partly to the subsequent eruption of the 

 igneous rocks just mentioned through the old red sandstone, 

 and the coal formation which rests upon it. 



4. The strata of sandstone are fully exposed on the shore 

 and in the cliffs from Rothsay to Ascog. South of Bogany 

 Point limestone appears interstratified with the sandstone, the 

 two rocks gradually passing into one another at the junction. 

 Dr. MacCulloch describes one bed — I noticed several others ; 

 but the beds being thin, of small horizontal extent, and con- 

 taining generally much siliceous matter, the rock is of little 

 oeconomical importance in this place. On the north side of 

 the small rocky promontory south of Ascog Mill thin courses 

 of nodular limestone traverse beds of brown shale. Subordi- 

 nate to the sandstone, the shale is of considerable thickness, 

 and appears again in the high banks above the road. 



The south side of the same promontory presents the follow- 

 ing strata: — The lowest bed is a fine-grained bluish-gray 

 nodular limestone; it is succeeded by a black bituminous 

 shale, containing veins of coal less than a quarter of an inch 

 thick. A bed of concretionary limestone rests on the shale, 

 the base or paste being a dark-coloured limestone, and the 

 concretions rounded lumps of the same rock, often of consi- 

 derable size. The upper part of the cliff is occupied by trap 

 in various prismatic forms. By the contact of the trap, both 

 the base itself and the imbedded lumps are so much altered 

 as very closely to resemble the trap itself. The trap rock 

 occupies a considerable area inland, and is upwards of 100 

 feet thick. Dr. MacCulloch has fallen into a slight error 

 regarding it: he says, " When examined on the shore it ap- 

 pears rather to pass through the sandstone than to lie over it; 

 but there is considerable obscurity in this place, as the lateral 

 junction of the two is concealed by a cavity filled with earth." 

 The junction is better exposed at present, probably in con- 

 sequence of the continued action of the sea ; and there can be 

 no doubt that the relative position of the strata is such as 

 above described. The trap reposes upon the sandstone, and 

 does not pass through it. 



5. The most considerable mass of limestone in the island 

 occurs on the south side of Kilchattan Bay. Its characters 

 are accurately described by Dr. MacCulloch, but he has mis- 

 taken its position. " This bed seems to lie over all the sand- 

 stone strata at this place, and to be the rock immediately in 



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