no PKOCEEDINGS OF THE GEOLOGICAL SOCIETT. 



associated ' ultra-basic ' varieties where the felspar almost disappears, 

 and the material consists mainly of ferro-magnesian minerals. The 

 more basic rocks are generally found towards the base of the vol- 

 canic series, where they appear as the oldest floWs. On the other 

 hand, in the Garlton Hills lavas of a much more acid nature are 

 met with — true sanidine-trachytes, which overlie the porphyrites 

 and basalts of the earlier eruptions. No adequate investigation 

 has yet been made of the chemical and microscopic characters of 

 these various rocks, regarded as a great volcanic group belonging to 

 a definite geological age, though many of the individual rocks and 

 the petrography of different districts have been more or less fully 

 described. I cannot here enter into much detail on the subject, 

 but must content myself with such a summary as will convey some 

 idea of the general composition and structure of this very interest- 

 ing group of rocks. 



(a) Augite-olivine Bocks (Picrites and Limhurgites). — At the base 

 of the Plateaux there are found here and there sheets of ' ultra- 

 basic ' material, some of which appear to be bedded with the other 

 rocks and to have flowed out as surface-lavas, though it may be 

 impossible to prove that they are not sills. Thus on the south 

 side of the Garlton Hills, at Whitelaw Hill, a dark heavy rock is 

 found to contain hardly any felspar, but to be made up mainly of 

 olivine and augite. 



(b) Basalts. — Among the sheets of the Plateaux, but especially 

 in the lower parts of the series, other basic lavas take, in some of 

 the districts, a conspicuous place. They are dark, often black, 

 usually more or less porphyritic, with large felspars, frequently also 

 large crystals of augite or olivine, and may be described as porphy- 

 ritic olivine-basalts, or more rarely olivine-free basalts or dolerites. 

 But they differ greatly in structure from the Tertiary basalts of the 

 Inner Hebrides and Antrim, for they rarely present any trace of the 

 ophitic structure so characteristic of the latter. Their groundmass 

 consists of short laths or microlites of felspar (probably labradorite) 

 and granules or small crystals of augite and magnetite, with some- 

 times a little fibrous brown mica. The large porphyritic felspars are 

 striped (probably labradorite), the augites are frequently chloritized, 

 and the olivines are generally more or less serpentinized. But in 

 some cases all these minerals are as fresh as in a recent basalt. 



Examples of these rocks may be seen among the lowest lavas of 

 most of the Plateaux, but they are especially abundant and remark- 

 able in the Campbeltown plateau of Argyllshire, where they have 



