Some Notes on a Bock allied to Limliirgite. 155 



XI. Some Notes on a Booh allied to Limbiirgite, near North 

 Berwick. By J. G. Goodchild, Esq., F.G.S., E.Z.S. 



(Read 20th April 1898 ; received for publication April 1899.) 



The coast from the West Links at North Berwick east- 

 ward past Tantallon Castle has long been celebrated for the 

 remarkably fine exposures of eruptive rocks which are there 

 laid bare. There are beds of both trachytic and andesitic 

 lavas at Kirkness and elsewhere ; an almost endless variety 

 of beds of tuff, showing every stage of gradation into rocks 

 of sedimentary origin ; one or more beds of geyserite ; necks, 

 intrusive sheets, and dykes of various kinds, from trachyte to 

 limburgite — some of them remarkably fresh — are beautifully 

 exhibited. There are also some examples of sand dykes, 

 evidently due to the deposition of sand in cracks and fissures, 

 formed, perhaps, as a result of earthquakes. 



In the case of a group of rocks which, as a whole, are 

 of so varied mineralogical composition, one would naturally 

 expect to find examples of rocks of types which do not fit in 

 very well with the currently-received petrographical classifi- 

 cations. Such, indeed, is the case here. These intermediate 

 forms are of especial interest to those geologists who prefer 

 to take a broad view of matters petrographical, because they 

 very well illustrate the fact that ISTature, in evolving the 

 different varieties of eruptive rocks, has been, so to speak, 

 careful not to weed out the forms that connect one type 

 with another. There has been, in other words, no process of 

 Natural Selection at work in this department of Inorganic 

 Nature, so that a rock-species, strictly speaking, can hardly 

 be said to exist. On the contrary, a perfect and complete 

 chain of varieties can be traced which connect the most 

 extreme types with each other by an infinite number of 

 intermediate gradations. 



A rock of this indefinite kind forms a rather conspicuous 

 feature on the shore about midway between North Berwick 

 and Tantallon Castle. One part of it is named the " Yellow 

 Man " on the Ordnance Six-Inch Maps of the district. It 

 occurs as a very irregular series of masses, which may be 



