part 1] AXNIVERtSAHY ADDltESS OF THE PRESIDENT. Ixxxvil 



activity attained a wide extension through the Highlands. Scattered 

 dykes of the same distinctive types as those of the Ayrshire dis- 

 trict are found on the isles and littoral of Argyllshire, in the 

 Glenelg district, Central Ross, Eastern Sutherland, and the Orkneys, 

 and volcanic necks on the north-eastern coast of Caithness repro- 

 duce all the characteristics of those of East Fife. 



Of more doubtful significance are the east-and-west dykes of 

 quartz-dolerite and tholeiite, which are scattered over the southern 

 lialf of Scotland and across the Border. They were intruded in 

 relation with the faulting of the country at or near the end of the 

 Carboniferous Period. Closely connected with them in the Midland 

 Valley of Scotland are numerous large sills of quartz-dolerite, 

 sometimes accompanied by granophyre ; and to the same group we 

 may assign, not only the Whin Sill of Teesdale, but probably the 

 Tremadoc sills and perhaps the intrusions of Carrock Fell.^ Quartz- 

 dolerite, though not itself an alkaline rock, has been claimed by 

 Mr. Dewey & Dr. Flett for the ' spilitic series,' and may possibly 

 sustain a complementary relation towards the types rich in soda ; 

 but more light on the question is needed before we can assign to 

 this group of intrusions its part in the Carboniferous system of 

 igneous activity. 



If now we glance back at the history of our area during Paleo- 

 zoic times, it appears, from the point of view that I have tried to 

 suggest, as one complete cycle of events. The culminating episode 

 is that of the principal Caledonian crust-movements, to Avhich the 

 preceding events led up, while the succeeding can be regarded as 

 their natural sequelae. That epoch marks the logical dividing- 

 point between Lower Palaeozoic and Upper Palaeozoic time. 

 Igneous action was an essential factor at numerous stages of the 

 cycle, but it assumed different phases and contrasted characters in 

 accordance with the changing stress-conditions. In particular, 

 we observe how, during the tranquil subsidence with which the 

 cycle began and ended, the magmas extruded and intruded were 

 invariably of alkaline nature, but calcic magmas took their place 

 at all times of special stress. Besides the great system of plutonic 

 intrusions which followed the main disturbance, the Palaeozoic cycle 

 included three minor igneous c^^cles, the Ordovician, the Lower Old 



^ The quartz-dolerites of Hawoswater and other parts of the Lake District 

 are older than the Caledonian crust-mo vementa. 



