SOME ANCIENT AND MODERN VOLCANIC ROCKS. 411 



5. Base Brown, Borrowdale. — The rock, the microscopic structure of 

 which is shown in figs. 11 and 12, is exceedingly compact and felstone- 

 like, and characterized by streaky lines resembling those in some 

 of the felstones of Snowdon (fig. 20). Every here and there a frag- 

 ment can be discerned, with the streaky lines of flow enveloping it. 



Under a high power the base is as clearly granular (fig. 11) 

 as in the test-specimen of ash (fig. 10) ; and in polarized light, with 

 crossed prisms (fig. 12), it is dark, with scattered faint points of 

 light, the broken fragments of altered felspar crystals and of felsite 

 standing out clearly just as in fig. 10. There is here, as in the last 

 specimen, the same collection of chloritic matter along lines of flow 

 (fig. 11), and in some cases the chlorite would seem to have crystal- 

 lized along bands in its frequent comb-like fashion. The streaky 

 character of this rock is shown in fig. 11, and is slightly seen also 

 in fig. 12. The structure may well be compared to that of the 

 slaggy felstone of Snowdon (fig. 20) ; but there is an absence of the 

 bands showing a felsitic structure, the base being all very closely 

 granular. 



The following is an analysis of this rock, beside which I will place, 

 for comparison, that of the Aran Mowddwy felstone, already given. 



Base Brown. Aran Mowddwy. 



Silica 69-673 



Alumina 14-492 



Potash 4-554 



Soda 3-017 



Lime.. 2-296 



Magnesia -324 



Ferrous oxide 2-784 



Ferric oxide *442 



Bisulphide of iron '410 



Phosphoric acid *343 



Sulphuric acid *205 



Carbonic acid -660 



Loss on ignition *800 



83-802 



7-686 



2-161 



4-229 



•896 



•109 



•408 



•111 



•191 



•089 



•017 



trace 



•301 



100-000 100-000 



"When the analysis of this altered and felstone-like rock is com- 

 pared with those of the lavas given on page 408, it will be seen that 

 the difference chiefly consists in an increased percentage of silica, 

 and decrease in that of lime, magnesia, and the iron oxides, while 

 the difference between it and the Aran Mowddwy felstone is much 

 more marked in several particulars. If I am right in believing that 

 this rock is really but an intensely altered fine ash, it would be 

 only reasonable to conclude that the composition would approximate 

 to that of the lava-beds, with such differences as might result from 

 a slightly different period of eruption* or from local circumstances. 



* The rock in question is considerably higher in the series than the lava-beds 

 of which the analyses are given. 



