408 



JT. CLIFTON WARD ON THE MICROSCOPIC STRUCTURE OF 



a somewhat different appearance, though the chemical composition 

 of the two appears pretty similar*. 



Brown Knotts. 



Silica 60-718 



Alumina 14-894 



Potash 2-354 



Soda 2-843 



Lime 6-048 



Magnesia 1-909 



Ferrous oxide 6-426 



Ferric oxide 1-405 



Bisulphide of iron -395 



Phosphoric acid '281 



Sulphuric acid *103 



Carbonic acid 1-660 



Loss on ignition 



•964 



100-000 



Iron Crag. 



59-511 



17-460 



3-705 



3-093 



5-376 



1-801 



4-926 



1-271 



•604 



•115 



•086 



1-569 



•483 



100-000 



3. Latterharroiv, near the foot of Wastwater (fig. 8). — The rock 

 now to be described occurs near syenite and granite, and among 

 highly altered ashes penetrated by veins of felstone from the neigh- 

 bouring syenite. Its lithological structure is that of a dark-grey 

 base with imbedded white felspar crystals and some soft green spots. 



When viewed microscopically, the structure is found to be some- 

 what variable. In many parts there is a decided network of acicular 

 felspar prisms, but in other parts only a number of greenish parti- 

 cles with patches of green mineral on a white hazy ground. There 

 is sometimes a distinct flow of the small crystalline needles around 

 the large crystals of felspar and altered augite, as seen in fig. 8, 

 with crossed Nicols ; in this figure there occur portions of two large 

 highly altered felspar crystals and some smaller fragments, together 

 with what appears like a pseudomorph after augite with its angles 

 much rounded. The base is not absolutely dark when thus viewed 

 under crossed prisms ; for when a J-inch objective is used, there are 

 seen to occur, between the dark parts formed by a diffused altered 

 mineral, numerous interspaces with a felsitic structure. Those parts 

 in which the acicular network is absent, show, under crossed Nicols, 

 the coloured-breccia reaction of a felsitic base. 



The felspar is throughout very much altered, and its species not 

 easy of determination ; some of it, however, is certainly orthoclase, 

 one example showing its characteristic twin structure. The grains of 

 magnetite are few; and only a little of the augite remains unchanged. 



General Remarks. — The preceding examples serve to show the 

 general character of these old contemporaneous traps. Many others 

 have of course been examined in the same way, and the general re- 



* I have little doubt that there are some few examples among the contem- 

 poraneous traps which more nearly approach true basalts than these, the 

 analysis of which would probably be of a more basic character ; but the above 

 are characteristic samples of the group as a whole. 



