ON THE GEOLOGY OF ARDROSSAN. 607 



grained dolerite with practically no olivine, and with the small idiomorphic augite and 

 hornblende crystals and the tiny laths of felspar set in an ill-defined felspathic matrix, 

 which is largely replaced by analcime and calcite. (PI. II., fig. 4.) At the upper 

 contact the rock is exceedingly fine-grained, much decomposed, and full of calcite. 

 There is no trace of fluxion structures even in the most minutely banded portions of 

 the sill. 



This rock, then, affords an excellent example of the differentiation of one and the 

 same magma into a lower basic and an upper felspathic portion. The composition of 

 the rock at the low r er contact does not materially affect the case, because although there 

 is there a recognisable increase in the amount of felspar, the rock as a whole is much 

 more nearly allied to the picrite than to the dolerite. There is, in fact, no question here 

 of differentiation into a central basic and marginal acid portions, for the transition 

 from picrite to dolerite can be traced in one direction only ; moreover, the differentiation 

 to which this is due must have taken place in the reservoir entirely before intrusion. 

 It is evident from the absence of fluidal structures in the finer-grained banded upper 

 portion that crystallisation did not begin until the whole mass was at rest. The 

 striation cannot, therefore, be due to the segregation of the various minerals into layers 

 during intrusion ; it must rather be assigned to a heterogeneity in the magma itself at 

 the time of intrusion. In this respect the Castle Craigs picrite presents considerable 

 analogy to the banded peridotites and gabbros of Skye, # and differs from the Blackburn 

 and Barnton picrites t in which the differentiation which has given rise to similar types 

 took place entirely after intrusion. Further, from the fact that this sill, like some of the 

 dykes of banded gabbro in Skye,| is asymmetrical with respect to the texture of its 

 upper and lower margins, we may conclude that there has been here also more than one 

 period of intrusion, and that the upper fine-grained and banded portion is somewhat 

 younger than the coarse-grained lower portion. That the one has very closely followed 

 the other from the same source is evident, however, from the very gradual manner in 

 which the dolerite everywhere passes down into the picrite. 



2. The Baths Gate Dolerite. 



This intrusive rock is pale gray or green in colour, and has probably been originally 

 a coarse-grained ophitic olivine dolerite. It is now, however, entirely decomposed and 

 more or less changed into a substance analogous to the white trap of the coalfields. It 

 has evidently been intruded into the midst of thinly-bedded carbonaceous shales, 

 sandstones, and ironstones ; and streaks and bands of these, especially of the sandstone, 

 are to be found traversing the decomposed rock, like veins of a later origin. These ribs 

 of sandstone, on fracture, are frequently black and lustrous, and under the microscope are 

 composed of scattered rounded quartz grains and black carbonaceous knots set in a 

 matrix of cryptocrystalline chalcedony. Evidently the sandstone has been partially 



* Tertiary Igneous Rocks of Skye, pp. 75, 90. t Ancient Volcanoes of Great Britain, vol. i. pp. 419, 450. 



+ Tertiary Igneous Rocks of Skye, p. 118. 



