281 



Mica-porphy rites appear to be less common than pyroxene-porphyrites. 

 They are generally reddish purple or brick-red in colour. The porphyritic 

 constituents are felspar (oligoclase ?) and biotite. The brown biotite is 

 usually more or less replaced by chlorite. Both the porphyritic felspars 

 and the ground-mass appear red by reflected light in consequence of the 

 large amount of ferric oxide scattered through the rock. The original 

 structure of the ground-mass is often very difficult to determine in con- 

 sequence of the large amount of alteration which has taken place. In 

 some specimens, however, it becomes distinctly micro-granitic or micro- 

 pegmatitic. This occurs however in dykes, and serves to connect the 

 andesites with the diorites. In the typical andesites and porphyrites 

 there is no recognizable quartz. The silica is latent, so to speak, in the 

 highly acid glassy base. It is important to note that the mica-andesites 

 are, as a rule, more acid than the pyroxene-andesites; so that, whenever the 

 ground-mass of a mica-andesite becomes holo-crystalline quartz makes its 

 appearance in considerable quantity. 



It is important to note that no representatives of the hornblende- 

 andesites or porphyrites have as yet been observed in the Cheviot district. 



When the Cheviot porphyrites become vesicular the cavities are 

 tilled with opal, chalcedony, quartz and green earth. Beautiful agates are 

 frequently found in the Coquet. These have been weathered out of the 

 amygdaloidal porphyrites. 



Bedded tuffs and agglomerates are associated M'ith the andesites and 

 porphyrites. They are composed of broken felspars, vesicular lapilli of 

 andesite or porphyrite, and fragments of similar rocks without vesicles. 

 The ground-mass, so to speak, of the tuffs which must have been composed 

 of the finer volcanic material has been so altered that Ave can no longer 

 recognize the nature of its constituent parts. The vesicles of the lapilli 

 are filled with chalcedony and quartz. Apatite, ferrite and viridite are 

 present in all specimens. 



The volcanic eruptions which gave rise to the Cheviot andesites must 

 have taken place in Lower Old Red Sandstone times. 



The Lake District. Mr. WARD < J > has proved that the Borrowdale 

 series of this district is mainly composed of volcanic rocks of intermediate 

 composition. He has also given us descriptions of the microscopic 

 characters of the rocks. Dr. SORBY (' 2 > has investigated many of the slates 

 and established the fact that they are composed to a very large extent of 

 fragmental volcanic material. 



Owing to the great age of these rocks and to the fact that they have 

 been affected by the post-Silurian earth-movements they are often found in 

 a highly-altered condition so that it is often extremely difficult to make 

 out their original characters. 



A typical series of lavas and tuffs occurs at Falcon Crag and Brown 

 Knotts in the neighbourhood of Keswick. This series has been described 



(1) Q.J.G-.S., Vol. XXXI. (1875), p. 406; and the Geology of the Northern Part of the 

 Lake District: Memoirs of the Geological Survey, 1876. 



(2) Q.J.G.S., Vol. XXXVI. (1880), Proceedings, p. 74. 



