THE SHAP GRANITK AND ASSOCIATED ROCKS. 299 



classification into rhyolitic, andcsitic, &c, ; but the rocks in question 

 seem to be made up largely of pyroclastic materials of the same 

 f>:eneral nature as the associated andesitcs, and give rise when meta- 

 morphosed to very similar results. They arc probably on the whole 

 somewhat more " acid " in composition, since they frequently enclose 

 rhyolite-tVagraents broken np by the explosive outbursts by which 

 the accumulations were produced, and mingled with the fine dust, 

 fractured crystals, and andesitic material. 



The non-metamorphosed rocks of this type may be studied in 

 Stockdalo, AVet Sleddale, &c., where the influence of the intrusion has 

 not been perceptibl)^ felt. The recognizable fragments are partly 

 crystals, partly pieces of lava. The crystals are similar to those 

 which occur porphyritically in the andesites themselves. They 

 very frequently lie with their length nearly at right angles to the 

 lamination of the finer matrix, indicating that they have been 

 dropped into their place [895]. This appears to be a characteristic 

 feature of pyroclastic rocks, especially those accumulated on land, 

 and affords a useful criterion in other districts where ashes and 

 lavas, chiefly of acid type, have been altered (not by thermo-meta- 

 morphism) almost beyond recognition. 



Some of the lava-fragments are of andesite. showing the usual 

 densely-packed felspar-prisms, and occasionally enclosing small vesi- 

 cles [875]. Others are of rhyolite, as already remarked. The 

 matrix of the mass is usually a finely-divided clastic material. Its 

 lamination is emphasized by the development along it of a pale- 

 yellowish or colourless sericitic substance which winds past the 

 enclosed fragments, and imparts a " schist "-like appearance to the 

 sections. Crystals, fragments, and matrix have undergone the 

 ordinary weathering processes, with the production of secondary 

 quartz, the usual pale-green product, a little magnetite dust, and 

 some calcite, which is more uniformly distributed than in the 

 weathered andesite lavas. 



Metamorphosed representatives of these rocks, which we may term 

 andesitic agglomerates and ashes, are met with intercalated among 

 the lavas at various horizons. The minerals produced are in 

 general those alreadj?^ described in the metamorphosed andesites. 

 Mica is the commonest of the coloured constituents. It is usually 

 of the highly pleochroic variety already noticed, giving a very deep, 

 rather greenish-brown colour for vibrations parallel to the cleavage- 

 traces. Sometimes it has less intense absorption, and is apparently 

 partly bleached [8751 ; or again, it is partially decomposed, giving a 

 green colour with secondary dust of magnetite. There is, however, a 

 rather different type of mica seen in some of the slides, having a 

 more ruddy brown colour and giving : 



/3 and y, chestnut-brown ; a, nearly colourless. 



This mica, when partially decomposed, loses its cleavage and 

 some of its pleochroism. Its characters would seem to indicate a 

 variety having a different chemical composition from the former, but 

 although the two types usually occur separately, they are in some 



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