Alexander Scott — The Crawfordjohn Essexite. 455 



opposition, and the winds blowing towards the Poles would be 

 increased in strength and produce a still further rise of temperature 

 on the earth's surface in high latitudes. 



The change from the archipelagic to continental conditions occurred 

 about the time when, for some reason or other, the whole of the 

 troposphere, and probably the stratosphere as well, became colder and 

 the snow-line was lowered. Glaciers, consequently, formed on high 

 mountain ranges and in high latitudes when the snowfall was 

 sufficient. With the passing away of the conditions which gave rise 

 to the Ice Age, glacial conditions disappeared almost entirely from all 

 except high mountain ranges of middle latitudes ; but owing to the 

 joining up of the islands on the continental platforms frigid conditions 

 remained over the Polar areas. 



The whole subject of the climatic conditions through which the 

 earth has passed is full of interest and difficulty. I have no wish to 

 appear dogmatic on the subject, for much will have to be learned 

 before any theory can be considered as probably correct. One thing 

 seems to be pretty clear, and that is that until a sound theoretical 

 reason can be given to account for the general winds of the earth 

 blowing as they do there is little chance of dealing satisfactorily with 

 the climatic changes which might result from geographical changes. 



VI. — The Crawfordjohn Essexite and Associated Rocks. 



By Alexander Scott, M.A., B.Sc. 



Introduction. 



ALTHOUGH the so-called Crawfordjohn essexite has been 

 mentioned several times in petrographic literature, no detailed 

 description of the occurrence has been given. 1 In 1888 Teall 3 

 described the main rock of the intrusion as an abnormal variety of 

 the N.W.-S.E. Kainozoic dykes and noted the porphyritic augite and 

 the abundance of felspar, olivine, and apatite. Lacroix, 3 in a general 

 paper on the teschenites, mentioned the occurrence of nephelite in the 

 same rock, which he described as an " olivine-teschenite, passing in 

 structure to a tephrite". Bailey, in the Glasgow memoir, 4 remarked 

 on the close similarity between the Crawfordjohn and Lennoxtown 

 rocks and classed them with the essexites on account of their chemical 

 similarity to the Brandberget rocks, 5 while Tyrrell," on account of 

 this similarity and also of the resemblance to the Carclout essexite, 

 included them in the late Palaeozoic alkaline group. 



The intrusion occurs on the west side of Craighead Hill, 7 about 

 one mile west of Abington Station and 2+ miles east of Crawfordjohn. 



1 No mention of it is made in the Explanation of Sheet 15 (Mem. Geol. 

 Surv. Scotland), 1871. 



2 British Petrography, 1888, p. 197. 



3 Compt. Bend., cxxx, p. 1273, 1900. 



4 Geology of Glasgow District (Mem. Geol. Surv.), 1911, pp. 113, 130. 



5 W. C. Brogger, Quart. Journ. Geol. Soc, 1, pp. 15-38, 1894. 



6 Geol. Mag. [5], IX, p. 121, 1912. 



7 The Craighead Hill, mentioned in The Silurian Bocks of Britain (Mem. 

 Geol. Surv.), i, 1899, pp. 462-3, 501, lies in the Girvan Valley, Ayrshire. 



