Norwegian Boulder in the Millstone Grit of Yorkshire. 57 



Region, the total destruction of the dark non-felspathic silicate 

 forbids one deciding this question.' 



Referring to the structure of the rock, he further writes : — 

 ' The structure resembles some " Ekerite-porphyries " from the 

 Christiania Region, and it is thus possible that this rock may be 

 an altered " Ekerite-porphyry " from Norway.' It does not, 

 however, agree with any of the ten thousand specimens of 

 alkaline rocks from the Christiania Region in the museum 

 under the direction of Prof. Brogger. 



While then a Scandinavian source cannot be definitely 

 assigned to this rock, it is so unlike any British rock with 

 which the author is acquainted, that he is disposed to think 

 that it may have been derived from some Scandinavian mass 



A 



Photo by] [R. Simpson. 



Polished surface of Pebble. 

 Natural size. 



which has long since disappeared. The age of the eruptions 

 which yielded the rhomb porphyry of Christiania, is Devonian, 

 the same as the Shap Granite, so that the specimen here 

 described may have been yielded by the denudation of the 

 erupted masses. In a similar way, pebbles of the Shap 

 Granite, which, of course, never reached the surface until 

 exposed by denudation, are found in the Carboniferous basal 

 conglomerate in the neighbourhood of Tebay. 



The only other areas where rocks of the rhomb-porphyry 

 type are known to occur are Mts. Kilima Njaro and Kenya, 

 East Africa; and in the neighbourhoood of Mt. Erebus in 

 Antarctica. 



: o : 



Prof. W. G. Fearnside's paper on ' Refractory Materials in South 

 Yorkshire,' read to the Midland Institute of Mining, etc., Engineers, 

 appears in The Quarry for January. 



1817 Feb. 1. 



