Reports cC- Proceedings — Ediiihiirgli Geological Society. 185 



He states, as a broad generalization, that there has been a gradual 

 decrease in the relative numbers of purple Zircons present in the 

 sedimentary rocks of Scotland in the order of their age from earlier 

 to later times. The purple Zircons rarely occur in Scottish granites, 

 and, when they do occur, they are to be regarded as derived from 

 the rocks through which the granite has been intruded. Dr. Mackie 

 finds that in certain Archaean gneisses of the North of Scotland, 

 the purple Zircons are unaccompanied by the normal Zircons of 

 igneous rocks, and he has arrived at the conclusion that these 

 Archaean gneisses are the original sources of all the purple Zircons 

 found in later sediments. He described also similar Zircons 

 from the " Banket " of South Africa, and from various Canadian 

 rocks, and raises the question as to whether purple Zircons may not 

 be characteristic of Archaean rocks in regions other than the North 

 of Scotland. 



. 2. " Note on the Exposure of St. Leonard's Crag, at No. 39 

 St. Leonard's Hill." By the President, Mr. T. Cuthbert Day, 

 F.C.S., F.R.S.E. 



There are several points of interest in connexion with this well- 

 known but seldom visited exposure of the basaltic sill of St. Leonard's 

 Crag. The chief among them is a peculiar line of fracture extending 

 for some yards in a nearly east and west direction. Much material 

 which would have furnished useful evidence has been quarried 

 away, Mr. Day supposed the fracture to be contemporaneous 

 with the intrusion, and that some of the sedimentary sandstones 

 above the sill were drawn in, crushed, and mingled with fragments 

 of some of the already partially consolidated basalt, the whole 

 being subsequently bound together by a final phase of the intrusion, 

 in which all the broken fragments were bound together by the 

 injection of porphyritic basalt, largely in the form of a peculiar 

 tachylyte. 



During the discussion which followed, Mr. E. B. Bailey supple- 

 mented this explanation, and suggested as a cause of the fracture, 

 that during the intrusion, there was an outburst of gas sufficiently 

 strong to fracture the partially solidified sill, and to lift the 

 presumably light cover, whereby a quantity of overlying loose 

 desert sand fell into the fissure, filling many vacant spaces ; a con- 

 tinuance of intrusion helped further to close the breccia as explained 

 above, while subsequent infiltration of calcite completed the task. 

 He also argued that the sill consists, broadly speaking, of a non- 

 porphyritic portion earlier than the explosion, and a porphyritic 

 portion subsequent to it ; the non-porphyritic was cooled very 

 markedly in the vicinity of the breccia by the time the porphyritic 

 portion was injected. 



3. Exhibits of (a) " Stones showing Sand-blasting and Insolation 

 from the Culbin Sands." By Allan Grant Ogilvie, M.A., B.Sc. 

 (6) " Garnetiferous Sand from above the Blackness (Limestone) 

 Coal." By H. M. Cadell, B.Sc, F.R.S.E. 



