Alexander Scott — The Crawfordjohn Essexite. 519 



been a tuff, as it contains rounded but clear crystals of felspar, 

 abundant dark aggregates of ferromagnesian crystallites and fragments 

 of a rock which was probably an andesite. The felspar has again 

 been recrystallized, and the shape of the.microlitic patches indicates 

 former phenocrysts of hornblende. In the andesite fragments small 

 clear felspar laths are found, as well as an altered bisilicate mineral 

 in which occasionally unaltered traces of augite appear. 



As a whole, the metamorphism differs from that developed round 

 the granites of the Southern Uplands, in the absence of such 

 characteristic minerals as andalusite and garnet. 1 Although the 

 sediments are so much obscured as to preclude any possibility of 

 examining their progressive metamorphism, it is very probable that 

 the altered aureole is narrow, extending to not more than a few feet. 



Nature and Age of the Intrusion. — Judging from the extent of the 

 exposures, the intrusion is not less than 250 yards long and 25 yards 

 broad. Indeed, the breadth is probably much greater as the most 

 southerly exposure is a very coarse-grained rock. If it were a dyke, 

 it might be expected that it would be seen in the Duneaton water, 

 400 yards to the north-west and 200 feet lower than the large quarry. 

 Although the grits and greywackes are exposed along the bed of the 

 stream, no trace of igneous rock could be found, either in the stream 

 or on the opposite hillside. Further, there is no evidence of any 

 continuation to the south-west, on Craighead hill or in the Clyde 

 Yalley. The nature of the igneous rocks, as well as the field 

 relations, indicates that the intrusion is probably a small elongated 

 boss. Its form is rather like that of the Lennoxtown essexite which, 

 although originally described by the Geological Survey as "an 

 irregular dyke of great thickness", 8 is classed in the Glasgow memoir 

 as "an elongated plug or small boss". 3 The Craighead intrusion 

 may be regarded as very similar, not only morphologically but also 

 lithologically, the only difference being the absence, so far as is 

 known, of any chilled margin in the Lennoxtown occurrence. 

 Although the Geological Survey express no definite opinion regarding 

 the age of the latter, they indicate that it is probably Carboniferous 

 or Permo-Carboniferous. There does not seem to be any doubt that 

 the Craighead intrusion has no connexion with the Kainozoic dykes, 

 and that it must be referred to the late Palaeozoic alkalic suite. Its 

 great petrographical resemblance to the Ayrshire representatives of 

 the latter group is strongly in favour of this, and, although the field 

 evidence only indicates a post-Llandeilo age, the lithological evidence 

 seems to preclude any connexion with either the Old Red Sandstone 

 or Kainozoic rocks. Adopting Tyrrell's suggestion that the 

 dominantly alkali rocks of late Palaeozoic age are Permo-Carboni- 

 ferous, 4 the Craighead intrusion may be referred to this epoch. 



1 Cf. M. I. Gardiner, Quart. Journ. Geol. Soc, xlvi, pp. 569-81, 1890 ; 

 J. J. H. Teall, loc. cit., pp. 632-49. 



2 Summary of Progress of Geological Survey for 1907-8, p. 55 ; ibid, for 

 1908-9, p. 45. 



3 E. B. Bailey, Geology of Glasgow District (Mem. Geol. Surv.), 1911, 

 p. 113. 



4 G. W. Tyrrell, Trans. Glasgow Geol. Soc, xiii, p. 311, 1909 ; Geol. Mag. 

 [5], ix, pp. 129-31. 



