OLDER rEKTDOTlTES OF SCOTLAND. 401 



preseuts so many peculiar characters that it appears to me to be 

 worthy of special description. In some respects it appears to differ 

 from any rock of the class that has previously been described. 



§ 3. The Scyelite (altered Mica-Hornblende-Piceite) of 

 Caithness. 



This rock is of so remarkable a character and affords so many 

 striking illustrations of the principles enunciated in the foregoing 

 pages, that a detailed description of it seems to be called for. 



It occurs as a boss rising above the thick mass of glacial gravels 

 on Aehavaiasdale Moor, situated in the Eeay country in the west 

 of Caithness, near to where that county borders on Sutherland. It 

 appears first to have attracted attention about fifteen years ago, 

 when Sir Eobert Sinclair and Mr. Tait noticed its peciQiar 

 appearance, and brought specimens of it to Thurso to submit to the 

 late Eobert Dick. 



Mr. David Gunn, of Thurso, and Mr. John Gunn, of Dale, near 

 Halkirk, have taken much interest in this peculiar rock and its 

 surroundings, and have sent specimens of it to different museums 

 and to geologists in various parts of the country. To the former 

 gentleman I am indebted for a carefully constructed plan, showing 

 the dimensions and positions of the singular rock-mass, with a series 

 of specimens taken from different parts of it ; to the latter I am 

 under the very greatest obligation for specimens of the material in 

 the best state of preservation and for much valuable information 

 collected with much care and patience. 



The boss of highly glittering rock, which is said to rise about 

 10 feet above the general level, is about 9 yards long from S.W. to 

 N.E. and about 7g yards broad from N.W. to S.E. It is surrounded 

 by a mass of disintegrated fragments derived from the same rock, 

 which extends over a nearly circular area about 25 yards in 

 diameter ; excavation to the depth of 5 feet in this disintegrated 

 material failed to reach the solid rock. The bright silvery scales 

 so abundant in the rock and the soil derived from it, together with 

 the covering of grass with which it is clothed, make the boss a very 

 conspicuous object in the midst of the heath-covered moor. 

 Prof. Heddle states that the rock which surrounds the boss on all 

 sides is a "syenitic gneiss ;" but owing to the thick covering of drift- 

 gravel the actual junction between the two rocks has not been 

 reached in any of the excavations that have been made. 



Carefully selected specimens of the rock were sent to Mr. Hugh 

 Eobert Mill, E.Sc, P.I.C, and by him were submitted to chemical 

 analysis in the laboratory of the University of Edinburgh. I have 

 that gentleman's permission to publish the results which he obtained. 

 Three more or less complete analyses of the rock were made, giving 

 the following results : — 



