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each other and to the belonospherites (pseudo-spherulites) and granospherites 

 is by no means well defined. In the glassy rocks we frequently find more or 

 less spherical patches of crypto-crystalline felsite, or isotropic micro-felsite 

 without radial or concentric structure. These are sometimes aggregated 

 together so as to form bands. In spherulites of this type the fibres instead 

 of radiating from one point frequently radiate from two or more points so 

 as to produce a complex structure. Again, it frequently happens that the 

 spherulites have been developed side by side, in which case the spherical 

 form of the individuals is more or less destroyed by mutual interference. 

 When the fibres do not radiate from a point, but from a line of greater or 

 less length, we have the structure for which ZIRKEL proposed the term 

 axiolite. The lithoidal or stony bands in many laminated rhyolites are 

 made up of spherulites and axiolites which have often suffered mechanical 

 deformation by movements in the magma after they have been developed, 

 and before the final consolidation of the rock. (1 > 



By an increase in the amount of this felsitic matter the glassy rock 

 passes into a true felsite. Sometimes a more or less marked fibrous 

 structure occurs in this felsite. When this is the case and when the fibres 

 radiate from a centre the spherulite gives a black cross in polarized light 

 with crossed nicols, and the character of the double refraction is positive, 

 that is the minor axis of depolarization at any point is coincident with the 

 radius of the spherulite. The spherulites of the Lea Rock may be quoted 

 as examples of this type. They usually occur as more or less independent 

 bodies in a glassy matrix and are generally more deeply coloured than the 

 rest of the rock. Thus the spherulites and felsitic bands in the Lea Rock 

 are stained a deep red by specks and flakes of ferrite. When examined with 

 a high power the fibrous structure appears to be due to the arrangement ol 

 the disseminated dusty particles. Distinct crystalline fibres cannot be made 

 out. In many felsites we find spherulites of a totally different character 

 from those above described. They are extremely minute, well-defined at 

 their margins and give a sharp, black cross. In ordinary light they show 

 a radial structure, and are frequently more transparent than the matrix in 

 which they lie. The character of the double refraction is negative ; in 

 other words the major axis of depolarization is coincident with the radius of 

 the spherulite at any point. Good examples of this kind of spherulite 

 occur in the " spotted " felsite of St. David's, and in many (not all) of the 

 spherulitic felsites of Arran. Spherulites of this kind sometimes form the 

 main mass of the felsite in which they occur ; at other times they are 

 aggregated in bands and patches. They are similar in appearance and 

 optical characters to the spherulites in hyalite. 



The conditions under which the double refraction of certain felsitic 

 rocks has been produced have been investigated by Messrs. RUTLEY and 

 ALLPORT who have proved that in many cases it is undoubtedly a secondary 

 phenomenon due to the devitrification of a glass. 



Glassy rocks of pre- carboniferous age are unknown, but their devitrified 



(1) SCEOPE. Geology of the Ponza Isles. Trans. Geol. Soc., Vol. II., 2nd weries. 



