676 



from the mineral structure of the rocks in which they were imbedded ; 

 or the rocks were assumed to be of the carboniferous period by the 

 species of the imbedded plants : whereas true geological reasoning 

 required that, anterior to either of the preceding conclusions, the 

 true position of the culm measures should be determined by actual 

 sections. 



The author then goes on to point out the difficulty of classify- 

 ing the vast series of schistose rocks below the old red sandstone 

 — from the great resemblance of their mineral type — from the 

 absence of well-defined beds of organic remains in many large re- 

 gions — and from their entire disappearance in the last members 

 of the descending series. The Silurian system is almost the only 

 exception to this remark ; and even this system is developed 

 in many parts of England without any distinct succession of natural 

 groups. The mineral type is on the whole much more uniform in the 

 great series under notice than in the secondary system of England ; 

 but the frequent absence of organic remains, and of any succession 

 of distinct groups, is compensated by the enormous scale of deve- 

 lopment, as shown in the natural sections : and the author concludes, 

 that it is not by hypothetical views and analogies, or by maintaining 

 one part of geological evidence at the expense of another ; but by 

 applying every kind of evidence in its proper place, and above all by 

 actual surveys and detailed sections, that we can ever hope to bring 

 into coordination the complicated phsenomena of which he is only 

 attempting to give a brief synopsis. 



TWO CLASSES OF OLD STRATIFIED ROCKS, &C. 



The author first notices the older stratified series of Scotland, and 

 divides it into two classes. 



(1.) The primary class (composed of gneiss, mica slate, quartz 

 rock, &c. &c.,) is largely developed in the Highlands. 



(2.) The second class (greywacke, greywacke slate, &c. &c.,) is 

 also largely developed in the Lammermuir hills, and in the whole 

 chain extending in the south of Scotland from St. Abb's Head to 

 the MuU of Galloway. It is shown, partly on mineralogical cha- 

 racters, and partly on the evidence of sections, that the rocks of the 

 former class are inferior to those of the latter. For a zone of slate 

 rocks (the roclies cMoriteuses et quarizeuses of Dr. Boue) is superior 

 to the crystalline slates of the Grampians, and is (at least pro- 

 visionally) placed on the same parallel with the earthy and me- 

 chanical slates of Lammermuir chain. Both the preceding classes 

 are shown to be inferior, and generally unconformable, to the old red 

 sandstone ; which in the northern part of Scotland was once grouped 

 with the primary class ; but in the geological map of Scotland is now 

 put in its true place. 



After giving a series of sections to connect the structure of the 

 Lammermuir chain with the adjacent parts of the north of England, 

 he then proceeds to describe, in general terms, the expansion of the 

 rocks of the second class through various mountainous tracts of 

 South Britain. The frontier chain of Scotland — the slaty series of 



