Natural History* 405 



firming the views formerly held out. Setting aside this minor 

 consideration, it is always useful to accumulate examples of 

 any geological fact ; particularly of such as, from their novelty, 

 or from their disagreement with former observations, are often, 

 for a considerable time, received with doubt or incredulity. 

 To multiply the places of access to such appearances is also 

 useful ; and I can only regret that I have riot here to refer to a 

 country more accessible, instead of being, as it is, more remote 

 than Aberdeenshire. 



Granite of various characters occurs in different parts of the 

 Shetland islands; where it displays, in a degree of profusion 

 not to be equalled through the whole of Scotland, all these 

 phenomena attending veins, and accompanying its contact with 

 the stratified rocks, which are, deservedly, objects of so much 

 attention to geologists, and which serve to throw so much light 

 on the nature and origin of this substance. But the most 

 entire and extensive tract is found in North Maven, extending 

 over a space which it would be useless to describe in words j 

 as, without a map, no definite idea could be conveyed of it. 

 There are, at least, two very distinct varieties in this district ; 

 and, it is not difficult to discover that they are of different eras ; 

 since veins of the one variety are found to penetrate into the 

 other, whereas the reverse never takes place. 



It is in one of these that the varieties, analogous to those of 

 Aberdeenshire, formerly described, are found ; and they present 

 a similar series of graduating specimens ; the whole being 

 evidently inferior to gneiss and the other primary strata of that 

 district, and, in many places, graduating into undisputed 

 varieties of the most ordinary granite. To detail the aspects 

 of these specimens, would be merely to repeat that which was 

 said in the former communication on this subject. I shall 

 therefore merely add, that from the ordinary syenitic granite, 

 consisting of hornblende, quartz, and feldspar, with or without 

 mica, a regular series may be traced, passing through numerous 

 modifications of greenstone, not differing from those of the 

 trap family, down to a perfect basalt. 



For the information of those who may be inclined to visit 

 the ground in question, I may add that the most convenient 

 situations for examining these appearances in detail, are in the 

 neighbourhood of Hillswick. 



2. On the Deposition of Carbonate of Lime in Wood. It is 

 well known that siliceous earth is deposited in many vegetables, 

 particularly in the grasses, in the bark of the Calamus Rotang, 

 and in that of Equisetum hyemale. The deposit known by the 

 name of Tabasheer, is a particularly conspicuous example of 

 this nature. The deposition of carbonate of lime is a more 

 rare occurrence ; yet it is found in many pears, and is very re- 



VOL. XL 2 E 



