40 MR STEVENSON ON THE GEOLOGY OF COCKBURNLAW, 



examples are seen in this district, it seems not improbable that the syenite of 

 Cockburnlaw and the Staneshiel is nothing more than greywacke, fused by the 

 agency of the molten granite, and the mineral characters of the two rocks thereby 

 blended together. The granite invariably assimies the aspect of syenite, as it ap- 

 proaches the greywacke. At Cockbnrn Mill dam, the greywacke is considerably 

 hardened, and dips to NNW. at high, but varying angles. Below the dam the 

 hardness increases, the planes of stratification become less distinct, while those 

 of cleavage grow more decided at eveiy step. All these symptoms of meta- 

 morphism increase as Ave approach the igneous rock, the texture of the greywacke 

 being changed to crystalline, and the size of the crystals increasing with the hard- 

 ness, imtil we arrive at a point, where it is impossible to decide from the appear- 

 ances presented, whether the rock should be considered greyiuacke or .syenite. Be- 

 yond this, it graduates into true syenite, which is divided by cleavage planes into 

 large rectangular blocks, arranged in the form of thick beds, having he same dip 

 and strike as the adjoining greywacke. The cleavage planes which run parallel 

 to those of the stratification of the greywacke (if, indeed, these are not merely 

 the original planes of stratification of the rock before it was converted into syenite), 

 are distinguished fi'om these nmning at right angles thereto, by being occupied by 

 veins of Iteavy-spar, associated with crystals of quartz. These veins run con- 

 tinuously in a WSW. to ENE. direction. As Ave proceed further into the mass, 

 the syenite becomes more ciystalUne, and passes by a regxdar and gradual transi- 

 tion, into the well characterized granite of the Staneshiel. Two varieties of gra- 

 nite are here seen intruding in the form of veins, which have evidently been 

 poured, in a molten state, into fissures opened in the syenite, dm-ing the process 

 of crystalhzation. One of these is a beautiful and regularly crystallized granite, 

 larger in the grain than is commonly met with in the adjoining hiUs. The other 

 is small grained, and rather soft, and seems to be connected with a dyke which 

 occurs in the bed of the river a Uttle above the dam, being identical in mineral 

 character. This dyke can be traced, when the water is low, for about 200 yards. 

 It is about five feet thick, and nins nearly NNW., but is frequently interrupted 

 in its course by the gi'eywacke which it crosses. It contains, in some places, 

 fragments of the latter rock, which it has detached and brought up with it, in its 

 passage through the strata. The adjoining strata have been partially fused, and 

 present in some jjlaces a syenitic appearance. The alteration extends to a con- 

 siderable distance from the dyke, the strata being extremely hard, and frequently 

 exhibiting contorted laminae. In the immediate vicinity of the dyke the planes of 

 stratification are very obscure, being, in many places, merely marked by veins of 

 spar. Both the dyke and the adjoining metamorphic rocks are traversed by 

 numerous veins of heavy-spar, together with a few of quaiiz. 



The summit of Cockburnlaw consists of beds of metamorphic greywacke, 

 which dip to NW. at an angle of about 65°. The metamorphism of the strata 

 increases as we approach the great body of granite, which lies immediately below 



