264 American Association. 



the annexed note of Mr. Salter, Avho unhesitatingly compares these 

 remains with those known to Mr. James Hall, yourself, and other 

 North American Geologists, as occupying the true Silurian posi- 

 tion of the calciferous sandrock and base of the Trenton Lime- 

 stones. 



It is of course most gratifying to me to find that the general 

 views of succession of the rocks of my native Highlands, indicated 

 so far back as the years 1826-7, — opinions then formed irrespec- 

 tively of zoological evidences, and simply from the physical rela- 

 tions of the rock masses, should have been thus supported by fossil 

 discoveries. 



North American geologists will, of course, have no difficulty in 

 understanding and admitting the conversion of Lower Silurian 

 sediments into quartz rocks, crystalline limestones, mica-schists, 

 chloritic slates, &c. ; since their own eastern coast ranges exhibit 

 such phenomena, some of which have been described and mapi^ed 

 by yourself. 



To the geologists of the old country this determination is of 

 the deepest interest ; for it gives them a key to unravel the real 

 age of large masses of the quartzites, limestones, chloritic and clay 

 slates, mica-schists and quasi-gneissic rocks (sometimes more 

 sometimes less metamorphosed) Avhich occupy vast wild tracts of 

 the Highlands of Scotland. 



The general order of the Scottish rocks is, therefore, pretty well 

 ascertained. The lowest known rocks are masses of granitoid 

 gneiss, on the upturned edges of which repose certain hard gritty 

 beds, and conglomerates, often of a red colour, which, in the early 

 days of our science, were confounded with the Old Red Sand- 

 stone. Now, however, that the existence of conglomerates at 

 diflferent levels in the Lower Silurian rocks of the south of Scot- 

 land has been demonstrated, (See Siluria, p. p. 156-160,) the old 

 views dependent on the mineral characters only, have been 

 swept away. The lowest, indeed, of the conglomerates on 

 the northwest coast of the Highlands may pass for the Cam 

 brian Rocks of the Geological Survey. Then follows in an as- 

 cending order, the series of quartzites, mica and chloritic schists, 

 &c., with included limestones, representing in ametamorphic con- 

 dition the Lower Silurian sediments. 



It is highly probable that the Upper Silurian rocks which ex- 

 ist partially in the south of Scotland, have no real equivalent in 

 the Highlands ; since the metamorphic rocks above adverted to, 



