204 H. Deicey Sf Dr. J. S. Flett— 



which in the slides made for the Geological Survey has very closely 

 the refractive index of the ordinary ray of quartz (l - 544), and in 

 convergent light it has positive optical sign. These tests exclude all 

 felspars more rich in lime than oligoclase-albite, and are better than 

 any methods depending on extinction angles. The only difficulty in 

 applying them is that the felspar is often filled with minute grains of 

 chlorite and epidote. We shall see that chemical analyses have in 

 several cases placed the identity of the felspar beyond question. 



The albite is sometimes very clear and transparent, suspiciously 

 so when we remember the highly decomposed state of the rocks in 

 which it occurs. At other times it has a milky translucency, while 

 in many rocks it is filled with infinitesimal grains of chlorite or 

 epidote. Hence there is reason to believe that in some degree the 

 soda-felspar may be secondary, and that possibly the rocks have been 

 albitized. Now albitization, or the replacement of other felspars 

 by albite, takes place in many ways, as might be expected in 

 a phenomenon so generally displayed by the older igneous rocks. 

 Apparently it is sometimes a result of ordinary processes of weathering 

 by which albite and calcite or epidote replace lime-soda plagioclase. 

 Among the best instances of this type the rocks of Bardon Hill in 

 Charnwood Forest may be adduced. Their felspars were zonal with 

 basic centres, and the interior of the crystals is now a meshwork 

 of albite enclosing many grains of epidote, while the margins are 

 nearly solid albite. Albite and epidote or zoisite may also arise from 

 lime-soda felspar through shearing, as in many of the amphibolites 

 derived by movement from the basic intrusive rocks of the North-East 

 Highlands of Scotland. But albite also replaces lime-soda felspar by 

 a process of albitization, as Bailey and (irabham (10) have shown, 

 which may be a juvenile, post-volcanic, or pneumatolytic mode of 

 alteration. 



In the Cornish pillow-lavas the felspars occur in several distinct 

 states. They may be entirely replaced by calcite, quartz, and 

 sericite, as in many lavas of Brentor. In other rocks the lath- 

 shaped sections of felspar are a network of albite with the interstices 

 occupied by secondary minerals such as chlorite and calcite. We can 

 hardly avoid the suspicion that what lime felspar was originally 

 present has been dissolved out, as in the rocks described by Termier 

 (11) and the gaps filled in with decomposition products. But in 

 a fair number of the rocks of this class the albite is clear and fresh, 

 by no means very full of inclusions, and has much of the appearance 

 of a primary mineral. Yet all the other components of these spilites 

 have been completely decomposed; no augite can be found in them, 

 and the matrix is a dense aggregation of secondary products. An 

 example of this class is furnished by the lava of Devonport Workhouse 

 near Plymouth (12). Its albite crystals are not zonal except that 

 their interior is rather spongy and full of secondary inclusions, 

 while their external portions are compact. 



In rocks of this type it must be conceded that albitization of 

 soda-lime felspar has occurred on a not inconsiderable scale. We 

 believe that this is to be ascribed to a pneumatolytic change 

 that affected the spilites very shortly after their effusion. It 



