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in detail by Mr. WARD. The lavas often assume a vesicular character at 

 their upper and under surfaces ; the vesicles now being filled with calcite, 

 chalcedony and green earth. The ground-mass is generally of a greyish 

 blue colour. Porphyritic felspars and soft dark spots, representing the 

 decomposed ferro-magnesian minerals may commonly be observed. The 

 porphyritic felspars are generally so much altered as to give only aggregate 

 polarization. Sometimes, however, the twin striation may be recognized. 

 In form and relation to the ground-mass they are precisely like those of 

 modern andesites. The porphyritic ferro-magnesian constituents are 

 almost always replaced by green alteration products. That they were 

 originally in most cases pyroxenes may be regarded as certain ; probably 

 enstatite and augite were both present. The typical ground-mass of these 

 rocks was, as Mr. WARD has shown, a " mikrolithenfilz." Minute acicular 

 microlites of felspar, pyroxene-granules, usually replaced by green alteration 

 products, and magnetite grains, form the greater portion of this ground -mass. 

 The alteration to which it has been subjected appears to have destroyed all 

 traces of interstitial glass. We may reasonably conclude, however, that 

 such a substance was originally present. The above facts shoAv beyond a 

 doubt that the dominant variety of lava was a pyroxene-andesite. The 

 widespread alteration to which the rock has been subjected now induces us 

 to term it a pyroxene-porphyrite. 



An interesting variety of this rock occurs behind the Lodore Hotel, 

 near Grange, in Borrowdale (see WARD). It differs from the above type in 

 containing no porphyritic constituents. The rock is of a greyish blue 

 colour and very compact. It often breaks with a conchoidal fracture. 

 Under the microscope it is seen to consist of minute lath-shaped felspars 

 arranged with their long axes approximately parallel to each other ; green 

 alteration products, probably representing minute augite-granules, and 

 small grains of magnetite. This rock is precisely similar in structure and 

 composition to the ground-mass of the normal andesites and porphyrites. 

 Among the secondary minerals present in the lavas of the Borrowdale 

 series we may mention epidote and granular spherie. 



Fragmental volcanic rocks form a large portion of the Borrowdale 

 volcanic series. The size of the component fragments varies within Avide 

 limits. Owing to subsequent alteration it is difficult to determine with 

 certainty the original characters of the fine-grained ash. An andesitic ash 

 of the present day is usually composed of minute angular fragments of 

 glass often bounded by concave outlines which represent fractured vesicles^ 

 broken felspar-crystals and crystals, either whole or fragmentary, of the 

 ferro-magnesian constituents. It is highly probable that many of the 

 Cumbrian ashes originally possessed a similar composition. If so their 

 characters have been to a very great extent obliterated. The broken 

 felspars may in many cases be recognized. The ferro-magnesian minerals 

 have, however, been in almost all cases replaced by green alteration 

 products. The minute glassy particles have been devitrified and their 

 characteristic outlines are not often recognizable. The coarser fragmental 

 rocks have preserved their original characters in a much more perfect 



