382 EECENT WORK Or THE GEOLOGICAL STJEVET 



The marked imconformability between the red sandstones and 

 quartzites detected by Professor Mcol was observed independently 

 by Sir Henry James, and described by him in a letter to Sir 

 Roderick Murchison, dated 26th July 1856*. 



The fossils obtained from the Durness Limestone by Mr. Charles 

 Peach were considered by Salter to have strong affinities with 

 certain Lower Silurian forms of North America, ranging from the 

 Calciferous Sand-rock to the Trenton Limestone f. This determi- 

 nation, confirming, as it did, Sir Eoderick Murchison's reference of 

 the limestones and quartzites of Sutherland to the Lower Silurian 

 system, gave that geologist a new impetus in his investigation 

 of the structure of the JSTorth-west Highlands. After devoting 

 parts of the summer of several successive years to the task, he 

 arrived at what he believed to be the true order of geological 

 succession in that region. The western gneiss, forming the ancient 

 foundation-stone of Britain, he correlated with the Laurentian 

 gneiss of Canada, and the red sandstones and grits with the Cam- 

 brian formation of Wales. The great succession of Silurian strata, 

 resting unconformably on the Cambrian sandstones and Laurentian 

 gneiss, were grouped by him in ascending order, (1) Lower Quartz- 

 rock, (2) " Pucoid-beds " with Serjnilites MaccuTlochii at the top, (3) 

 Limestone with fossils, (4) Upper Quartz-rock followed in places by 

 (5) an Upper Limestone passing upwards into micaceous flagstones, 

 chloritic schists, and gneissose beds, covered unconformably by the 

 Old Eed Sandstone. Prom this unbroken sequence he inferred that 

 the Highlands are mainly composed of metamorphosed Silurian 

 strata. 



In his various papers Murchison maintained that the prevalent 

 strike of the western gneiss is N.W. and S.E., while that of the 

 eastern schists is l^.E. and S.W., and further, that this difference 

 is made still more apparent by distinct lithological characters. He 

 called special attention to the contrast between the hornblendic 

 gneiss in the west of Sutherland and the micaceous flaggy strata 

 overlying the limestones and quartzites to the east of Assynt, 

 Loch More, and Loch Eriboll. He naturally held that such a 

 difference in strike and lithological character implied a different 

 geological horizon. It is due to his memory to recognize how 

 clearly he saw the impossibility of accounting for the superposition 

 of the eastern or upper gneiss on the limestone by supposing it to 

 be the western or fundamental gneiss brought up again by mere 

 ordinary faulting or inversion. He was mistaken, as we now 

 know, in regarding this superposition as a normal stratigraphical 

 sequence. Eut the mistake was hardly avoidable at the time, and 

 was shared in at first by Nicol also. Indeed it could not be com- 

 pletely cleared up until laborious and detailed mapping had been 

 undertaken. To Murchison's mind the fact of prime importance in 

 the geological structure of the North-west Highlands was the position 



* Memoirs of Sir E. I. Murchison, by A. Geikie (1875), vol. ii. p. 213. 

 t Eep. Brit. Assoc, for 1857, p. 83 ; Quart. Journ. Geol. Soc. toI. xv. p. 374 

 et seq. 



