an:j?iveksary address of the peesideijt. 



Ill 



recently been mapped for the Geological Survey by Mr. E,. G. Symes, 

 and in that of the south end of Bute, where, together with an inter- 

 esting series of dykes and sills, they have been surveyed by Mr. W. 

 Gunn. But even where, as in the Garlton Hills, the lavas are for 

 the most part somewhat acid in composition, those first poured 

 out, which form the lowest band, include some typical olivine- 

 basalts. 



The following analysis shows the composition of a specimen from 

 the Garlton Hills : — 



Analysis of Olivine-hasalt from the Garlton Hills. 

 By Mr. J. S. Grakt Wilsoi^. 





SiO,. 



AI2O3. 



Fe,03. 



FeO. 



MnO. CaO. 



MgO. 



K2O. 



NagO. 



H2O. 



Total. 



Kippie Law, ) 

 Spec. grav. 2-8 f 



46-01 



19-19 



5-91 



6-75 



0-19 8-68 



6-Sl 



1-20 



3-27 



3-07 



101-08 



(c) Porphyrites {Andesites). — These are the most abundant lavas of 

 the Plateaux. They occur in every district, and usually form the 

 main constituents of the pile of volcanic material. They vary in 

 colour from a pale pinkish grey, through many shades of red, purple, 

 brown, and yellow, to sometimes a dark green or nearly black rock. 

 Their texture ranges from almost semi-vitreous through different 

 degrees of compactness to open cellular slaggy masses. Generally 

 through their base porphyritic felspars are abundantly disseminated, 

 sometimes in large, flat, tabular forms like those of the Lower Old 

 Bed Sandstone already referred to. The amygdaloidal kernels 

 consist of calcite, zeolites, chalcedony, or quartz. It is from the 

 amygdaloids on either side of the Clyde that the fine examples of 

 zeolites have been chiefly obtained for which the South of Scotland 

 has long been famed. 



Under the microscope these rocks present the usual fine aggregate 

 of felspar microlites, with granules or crystals of magnetite and 

 sometimes pyroxene. The porphyritic felspars, often so large and 

 well defined, generally contain inclusions of the groundmass. 

 Occasionally some of the large por])hyritic constituents are augite, 

 or pseudomorphs after that mineral. The alteration of the rocks 

 has oxidized some of the iron ore and given rise to the prevalent 

 purplish and reddish tints. 



