THE SHAP GRANITE AND ASSOCIATED ROCKS. 303 



nodular, — an arrangement noted in some other localities also. This 

 lumiuated rock has a typical microsplicrulitic structure, being almost 

 entirely built of densely-packed minute spherulites, each of which 

 gives a distinct black cross when a section is examined between 

 crossed nicols [861]. In some places the growth, instead of being 

 centric, is linear, and then follows the lines of flow. Slighter dif- 

 ferences in structure in different parts of the slide also follow the 

 fluxion-lines. A beautiful flgure of this rock, showing the micro- 

 spherulitic and perlitic structures, has been given by Mr. Teall *, 

 and the same rock has been described and figured by Mr. llutley t. 

 The latter author has expressed the opinion that the spherulitic 

 structure is here an effect of devitrification subsequent to the 

 perlitic cracking; but we are unable to see that he has given any 

 reasons for this view. The practice of assigning a secondary origin 

 to special structures in the older acid lavas has perhaps been 

 pushed to excess in some quarters. In the AVestmorland rhyolites 

 there are traces of perlitic fissures traversing rocks which are now 

 microcrystaUine, and other appearances pointing to the alteration 

 of an originally glassy mass ; but we find nothing to suggest that 

 the spherulitic and allied structures are of formation posterior to the 

 consolidation of the lava ; and the breaking up of the vitreous 

 material of the rocks examined seems to have been in manj' cases a 

 chemical, not merely a molecular change. 



In addition to occasional small crystals of quartz and felspar — 

 mostly plagioclase — the only original minerals found in these rhj'o- 

 litic lavas are scattered magnetite-crystals, and very rarely prisms 

 of apatite [802]. Probably a little augite or biotite formed part of 

 the original rocks, but a few scraps of the usual pale-green decom- 

 position-product are the only thing to indicate the former presence 

 of these minerals. Another secondary constituent is a yellowish- 

 brown filmy mineral, like sericitic mica, which usually occupies 

 perlitic cracks. 



A frerjuent type of alteration in the rhyolites, shown in many of 

 our specimens, is what we may conveniently term " silicification." 

 The groundmass of rocks so affected presents a finely crystalline 

 appearance, and consists mainly of quartz in a fine-grained mosaic, 

 passing in irregularly disposed patches into a rather coarser grain,. 

 Included crystals of felspar are frequently pseudomorphed by a 

 similar quartz-mosaic, and the process is sometimes made very evi- 

 dent by portions of the twinned crystals remaining still unaltered 

 [802]. The occurrence of these silicified rhyolites shows no relation 

 to the proximity of the granite, and we do not ascribe it to meta- 

 morphism by the agency of the intrusion. Identical phenomena 

 are observed in various localities in Caernarvonshire remote from 

 any igneous intrusion, and Miss l^aisin j has suggested a " percola- 



* ' British Petrography ' (1888), pi. xxxviii. 



t Quart. Jouni. Geol. Soc. vol. xl. (1884) p. 345, \A. xviii. fig. 0: Mem. 

 Geol. Surv. (1885), ' The Felsitic Lavas of England and ^Yales,' p. 12, pi. ii. 

 fig. 1. 



X Quart. Jouru. Geol. Soc. vol. xlv. (1880) p. 2()7. 



