140 NOTICES OF NEW BOOKS. 



and trap rock, and the enormous preponderance of the metamor- 

 phic rocks (occupying nearly 75 per cent, of the whole area of the 

 country) give at once a distinctive geological character, and have 

 principally influenced the general physical features of North 

 Britain. 



The speculations of the author concerning the Geological age 

 of these extensive metamorphic rocks seem to point to the Silurian 

 period as that during which they were deposited in a sedimentary 

 slate ; and since they underwent their great changes and probably 

 became metamorphic before the Old red sandstone was accumulated 

 in unconformable layers upon them, a more modern date is not 

 at all probable. It is a remarkable fact, however, that while 

 in Scotland large fragments of the more distinctly metamorphic 

 rocks are frequently found, those of granite, gneiss, and mica slate 

 are nowhere met with. 



The Old red sandstone of Scotland has been the subject of 

 special description by Mr. Hugh Miller, and the present work 

 does not appear to add to our knowledge concerning it. The 

 author indeed retains as Old Red the beds of Red sandstone con- 

 taining footsteps at Corncockle Muir, respecting which Pro- 

 fessor Sedgwick and Mr. Murchison arrived at the conclusion 

 that it is New red ; and he considers that the Old red passes 

 into the red sandstones of the coal measures by imperceptible 

 gradations. With regard to the latter series Mr. Nicol leans 

 to the opinion of the coal having been formed in a shallow marshy 

 lake, alternately submerged and lifted to receive the deposits of 

 sand and clayey mud. The proportional superficial extent of the 

 coal measures in Scotland is stated as amounting to one-seven- 

 teenth of the whole area (in the whole of Britain it is calculated 

 at one-twentieth), and as being greater in actual extent than that 

 of all the coalfields of France together. 



The author offers a few observations concerning the alterations 

 of level during the most recent geological period, the marks of 

 which are evident in Scotland ; and he alludes also to the earth- 

 quakes still occasionally felt. These latter are indeed more nu- 

 merous than would be supposed, since Mr. Milne informs us that 

 within the last century (from 1732 to 1839) not less than 139 have 

 been noticed, although it would appear from the extreme irregu- 

 larity of their direction and the difference they exhibit in the ex- 

 tent to which they reach, that they are in a great measure local, 

 and depend therefore on partial causes. This is not always the 

 case however, as appears from the fact that the great earthquake of 

 Lisbon in 1755 was felt at Loclmess, and that in 1839 a shock took 

 place which was felt simultaneously over two thirds of Scotland. 



This work by Mr. Nicol appears to contain a good summary of 

 the main facts recorded concerning the Geology of his country, his 

 principal object having been to describe the mineralogical pecu- 

 liarities and the details of the structure of the older rocks. 



D. T. A. 



