156 Proceedings of the Royal Physical Society. 



trative of the ores mined in the district were exhibited ; 

 also some possessing unusual associations of minerals — a 

 specimen, showing granules of free gold in hard carbonate 

 of lead, attracting much attention, from the fact that it was 

 the only specimen ever discovered in the district exhibiting 

 this conjunction. Mr Gunn exhibited specimens of tellurium 

 ores of remarkable beauty, and a sample of zinc blende mined 

 in large quantity in Pitkin County, which, contrary to the 

 opinion generally held by miners, contained large quantities 

 of silver. The second portion of the paper dealt with the 

 San Juan district, and, after indicating the peculiar disad- 

 vantages under which this district laboured for the first few 

 years of its existence, proceeded to describe its geology, 

 which he stated to be Trachyte, overlying rocks of Carboni- 

 ferous and Devonian age. The mineral is found in true 

 fissure veins of great width, chiefly composed of quartzose 

 matter, but usually carrying one or more grey streaks, from two 

 to six feet wide, composed of galena, fahlerz, and sulphurets of 

 silver and gold. Some of the mines produce beautiful filaments 

 of native silver, and one of the specimens showed a very unusual 

 association — viz., fine filaments of silver on grey copper. 



XVI. On Boulder-Glaciation. By Hugh Miller, Esq., 

 A.K.S.M., F.G.S., Honorary Member of the Tyneside 

 Field Club. [Plate YL] 



(Read 23d April 1884.1) 

 TABLE OF CONTENTS. 

 Introduction. — The Till not Structureless. 

 L Glaciation of the Upper Surfaces of Boulders in situ. 

 Previous Observations. 

 Pavement-Boulders in the North of England. 



,, ,, near Edinburgh. 



Adjustment of the Boulders under Glaciation. 

 Their Striee. 



Their Occurrence at all Depths in the Till. 

 Boulder-pavements. 

 Conclusions. 



1 Part n. has been entirely written since this date. An abstract of this 

 part (on Fluxion-Structures) has been communicated to the meeting of the 

 British Association at Montreal. 



