55 



for some distance, and again to separate ; a narrow dike branches 

 off into several filaments, which unite again portions of the rock 

 traversed are often found entangled in the dike ; and these, as well 

 as the contiguous strata, present the usual metamorphic effects 

 recognized as due to igneous action, besides others of a peculiar 

 and exceptional character, to which we shall now allude. 



The Kilchattan limestone is altered in a remarkable manner by 

 a large dike, crossing it nearly in the direction of the dip, and 

 the effects are well seen at the eastern side of the quarry. Along 

 the plane of contact with the dike the limestone is altered to the 

 state of a granular saccharine marble, which on the application of a 

 slight pressure crumbles into a fine powder. This is succeeded by a 

 hard crystalline marble, the crystals appearing in distinct flakes. 

 Between this and the first change, which is one of simple induration, 

 there are many gradations. Similar effects are common at the 

 contact of limestone with plutonic rocks ; in some localities they are 

 accompanied by other singular changes of a chemical nature. Mag- 

 nesia, and sometimes silica and alumina, are introduced into the 

 composition of the limestone, so that simple carbonate of lime 

 becomes a double carbonate of lime and magnesia. The source 

 whence this magnesia has been derived has occasioned much dif- 

 ference of opinion among geologists. Some imagine that it has 

 been transferred from the plutonic rock to the limestone ; while 

 others hold that, as fractures and dislocations of the earth's crust 

 accompanied the eruption of these plutonic rocks, gaseous exhala- 

 tions might find their way from beneath, and introduce carbonate 

 of magnesia and other substances into rocks near the surface. In 

 confirmation of this view Mr. Phillips has shown, in his Geology of 

 Yorkshire, that " common limestone is dolomitized by the sides of 

 faults and mineral veins far away from igneous rocks of any kind ;" 

 and some distinguished chemists have expressed their belief that 

 carbonate of magnesia may be sublimed by the action of great heat. 

 (Rep. Brit. Assoc. for 1835, Trans. Sect., p. 51 ; Phillips's Geology, 



or three times the width, enters near it, crossing from Ascog, and visible in several 

 eminences in the interior. The Eothesay mineral water, which has acquired some repu- 

 tation for efficacy in rheumatic, cutaneous, and glandular complaints, rises in the former 

 of these two dikes, at Bogany point. The gallon'of 277-274 cub. inches contains, accord- 

 ing to the analysis of Dr. Thomson, com. salt, 1860'73 grains; sulph. lime, 125-20; 

 sulph. soda, 129-77; mur. magnesia, 32-80; silica, 14-39; sulphuretted hydrogen, 17-4 

 cub. inches. Both dikes and this mineral water are noticed at some length in Wilson's 

 Guide to Rothesay and the Island of Bute, an excellent and neatly illustrated little 

 work, full of interesting historical and antiquarian matter, but unfortunately, extremely 

 meagre on the geology. 



