Vol. 57.] SILURIAN [?] EOCKS IN FORFAKSHIEE, ETC. , 313 



(iii) The appearance of an upward succession at this line of junction 

 is alwa3'S deceptive, the boundary being marked by a great 

 line of displacement or overtbrust which has been proved to 

 extend for many miles. 



(iv) The apparent passage from the non-crystalline newer rocks 

 (Silurian?) into the Highland schistose series is equally 

 deceptive. 



(v) The crushing accompanying this thrust never extends more than 

 a few yards into the Highland Series, owing to the induration 

 of the rocks of that series by previous metamorphism. 



(vi) The position of the major thrust truncating the so-called Silurian 

 rocks on the north and that of the later great boundary- 

 fault skirting the Old Red Sandstone have been determined by 

 the outer limit of the great aureole of crystallization of which 

 the South-eastern Highlands form a part. The harder crystalline 

 schists to the north-west have snapped off, under the influence 

 of enormous pressure, from the softer portions now covered by 

 newer rocks to the south-east. 



(vii) Though the rocks have been much sheared, they have not been 

 changed into crystalline schists. In particular, the sheared 

 basic igneous rocks are never altered to hornblende-schists. 



Discussion. 



Sir Archibald Geikie stated that he had had occasion to examine 

 the Author's evidence with him on the ground, and had no hesitation 

 in agreeing with the conclusion arrived at hy Mr. Peach and 

 Mr. Home that the series of jaspers and igneous rocks wedged in 

 along the Kincardineshire border of the Highlands, between the 

 metamorphic rocks and the Old Red Sandstone, bear the closest 

 resemblance to the Arenig Group of the Southern Uplands, and are 

 in all probability of the same age. The Author, however, had 

 detected in that region a younger group of strata (Margie), which 

 may belong to some later part of the Silurian System. The Arenig 

 Group had been detected at other places along the Highland Border : 

 by Mr. Clough between the Tay and Loch Lomond, and more 

 recently by Mr. Gimn in the Island of Arran. Though the speaker 

 had gone over the ground with the Author, he had not been able to 

 detect any section showing indications of such a thrust-plane as the 

 mapping would appear to require. jS'or in the tracts farther to the 

 south-west had it been practicable to draw any hard-and-fast line, 

 between the Arenig Group and the metamorphosed clastic rocks of 

 the Highlands to the north of them. It is possible that, while 

 a thrust-plane may be shown by the mapping to exist in Kincardine- 

 shire, there may be no such break in the tract between Callander 

 and Loch Lomond. In Arran, too, it has been found impossible 

 to trace a definite boundary between the Arenig Group and the 

 contiguous schists. 



The questions involved in this paper are of the utmost interest 

 and importance, both as regards the structure and history of the 



