Outlines of Geology. 255 



disturbed by porphyritic dikes ; and near Gow's bridge, a dike 

 of greenstone disturbs and contorts the primary slate of the Hut- 

 tonians, and contains a mass of embedded marble*. Hereabouts 

 there is so much confusion and irregularity, such a variety of rocks, 

 and such various inclinations of their strata, that neither descrip- 

 tions nor drawings can give any thing but a remote and inade- 

 quate idea of their groupings and assemblages. 



The bordering hills are frequently in part composed of granular 

 quartz, and when this forms their summit, they are character- 

 istically conical, as seen in Cairn-Toockie, and some others. 



And lastly, dolomite, or magnesian marble, is here seen passing 

 in a few places, and in detached spots, into a rock which bears all 

 resemblance to serpentine. 



I have enumerated these facts, which are shewn in the valley of 

 the Tilt, because much of the fair and legitimate argument that 

 may be founded upon them applies to the phenomena exhibited 

 by the granitic or elvan veins of Cornwall ; they will lead us to 

 doubt the quiet crystalline deposition of the granite and its 

 associate rocks, and may perhaps justify us in inferring, with 

 Dr. Hutton, that they are of another source and parentage. 



St. Michael's Mount, situated in Marazion Bay, on the S. coast 

 of Cornwall, is also an interesting geological object, and shews us 

 granite and mica-slate, not merely at their junction, but the 

 latter rock is singularly traversed by granitic veins, which appear 

 to break up the superincumbent slate, and to penetrate and 

 harden it. Crystals of tin, quartz, and apatite, and small topazes 

 are also found in the veins of this rock. 



Exclusive of Cornwall and Devon, there is little granite in 

 England. The Malvern Hills, Mount Sorrel in Leicestershire, 

 and a few of the ridges of Cumberland and Westmoreland, afford 

 us specimens of this rock, but they present nothing sufficiently 

 remarkable to be further dwelt upon at present. In the Isle of 

 Man, and in Anglesey, granite is associated with clay-slate; and 

 near Gwyndy in Anglesey, the points of granite curiously protrude 



+ See the plates annexed to Dr. Mac Culloch'a paper in the Geol. Trans* 

 actions. 



T 2 



