10 



BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. 



[Memoirs National 

 [Vol. XIX, 



Hill, '12, p. 21.) This series of intrusions was followed by the injection of troctolite and later 

 of gabbro, with flaser or massive structure, forming innumerables dikes in the serpentine, and a 

 large mass extending beyond the limits of the visible serpentine. After a considerable period 

 of cooling there came the intrusion of olivine-diabase, and a set of composite intrusions of par- 

 tially mixed doleritic and granitic magma forming the streaky masses known as the Kennack 



gneisses, followed immediately by 

 small laccolitic masses of granite. 

 The extremely metamorphosed char- 

 acter of many of these rocks is ex- 

 plained as the result of the orogenic 

 pressure suffered while still hot. 2 



Early Paleozoic. — The chief 

 epoch of igneous activity in this era 

 was that commencing in late Cam- 

 brian times and extending into the 

 early Silurian. Rocks of the spi- 

 litic suite, both flows and intru- 

 sions, were developed during the 

 steady deposition of radiolarian sedi- 

 ments in a geosyncline. These now 

 appear along each side of the Central 

 Valley of Scotland. The northern 

 series stretching from Stonehaven to 

 Arran appear to be, in part at least, 

 of Upper Cambrian age (Campbell, 

 ' 13) , while those to the south stretch- 

 ing from Ballantrae to Peeblesshire 

 are Arenig (Peach and Home '97). 

 Jehu and Campbell ('17) have descri- 

 bed the northern series at Aberf oyle. 

 Here there are Upper Cambrian or 

 passage-beds into the Ordovician, 

 consisting of black shales and cherts 

 containing radiolaria, graptolites, 

 and brachiopods, with interbedded 

 spilites,and intrusive albite-dolerites, 

 " serpen tinized albite-gabbros," and 

 dunite serpentine, more or less 

 changed to a ferruginous dolomite. 

 The serpentine shows evidence of in- 

 tense shearing and has apparently 

 shared in the movements which in- 

 duced the f oliation in the rocks to the 

 north. In the region i m m ediately 

 north of Ballantrae there is a marked unconformity between the Ordovician and Silurian rocks, 

 though only 12 miles to the south of Ballantrae they are quite conformable. During the crust 

 movement indicated by this unconformity, approximately stratiform masses of pendotite and 

 gabbro were thrust into the Arenig rocks. The intrusive character of these was first proved by 

 Professor Bonney (78), and they are sharply distinct from the associated spilitic rocks; the-gab- 

 bros and serpentines are in separated masses, the latter being the older. The pre-Silurian age of 



■ Id the above account the opinions of Dr. Flett, who guided the writer over the region, have been adopted, as was done in Dr. Barker's address 

 ('17). Nevertheless attention should be directed to the divergent views urged by Prof. Bonney ('14). 



Flo 



1.— Sketch-map, showing distribution of Older and Younger Igneous Rocks in 

 Strathbogie and Lower Banffshire, titter Read.) 



