156 Geological Society : — 



The main portion of the paper deals with the petrology of the 

 igneous rocks, which occur as laccolites and sills, as dykes having- 

 for the most part a north-easterly direction, and in volcanic necks. 



The chief rock-types present in the area described may be 

 summarized as follows : — 



Porphyritic and non-porphyntic sanidine - traclrytes, quartz - 

 trachytes, riebeckite-felsites, quartz-porphyries, basalts, and volcanic 

 agglomerates. The sanidine-trachytes' present a number of varieties, 

 characterized either by the presence of fresh riebeckite, by the 

 presence of segirine-augite and olivine, or by the absence of any 

 fresh f erromagnesian minerals. The more acid types described may 

 be regarded as standing midway between the phonolites and tra- 

 chytes of the same age which occur south-east of Hawick and those 

 of East Lothian. 



In addition to the description of the various rock-types which 

 occur as isolated sills and dykes, the geology of the Eildon Hills is 

 discussed in some detail, and a petrographical account is given of 

 the rocks in that region. The author has traced as far as possible 

 the distribution of the various rock- types, and has come to the 

 conclusion that the Eildon Hills form the eastern section of a 

 composite, complex laccolite. A basaltic neck has been met with 

 in the Eildon -Hill complex, and appears to cut through the more 

 acid intrusions which form the main portion of the Eildon Hills. 

 The record of riebeckite in the rocks of this region has been 

 considerably extended. 



The salient features of the suite of rocks described are the high 

 content of alkalies, and the presence of soda-bearing minerals such 

 as riebeckite, segirine-augite, primary albite, and soda-orthoclase. 

 Nepheline, however, though its presence might be expected in some 

 instances, was found to be absent from most of the rocks. 



2. ' Correlation of Dinantian and Avonian.' By Arthur 

 Vaughan, M.A., D.Sc, F.G.S. 



March 25th.— Dr. A. Smith Woodward, F.K.S., President, 

 in the Chair. 



Prof. J. W. Judd, C.B., F.E.S., gave the following general 

 account of the geology of Rockall : — 



'Eockall is a small isolated rock in mid- Atlantic, lying 184 miles west of 

 St. Kilda ; it has a circumference of only 100 yards and a height of 70 feet, 

 and, except in the very calmest weather, is quite inaccessible. It is the haunt 

 of sea-birds and, with its whitened top, resembles a sailing ship, for which it 

 has often been mistaken. The rock rises from a bank (the "Eockall Bank ") 

 upon which there are several dangerous reefs. 



' More than 300 years ago it was reported that a large island occupied the 

 site of Eockall, and, for a hundred years or more, all Atlantic charts repre- 

 sented this island, which was named " Busse Island," with a number of other 

 islands and islets, as present in the North Atlantic. Taking these supposed 

 facts in connexion with the famous classical stories of an " Atlantis," the 

 theory was often advanced that the North Atlantic was an area of sub- 

 sidence, and that the reported islands — and, in the end, Eockall — were the 



