WARKEN. — ALKALI-GRANITES AND PORPHYRIES. 245 



almost an entire crystal, appears also homogeneous. A great part of 

 the feldspar, however, generally shows a very fine lamination, (crypto- 

 perthitic) which is parallel to the usual direction of the perthite inter- 

 growth. As the margin of the original crystal outline is approached, 

 the lamination becomes more distinct and two sets of lamellae can be 

 made out whose optical properties are those of albite and microcline. 

 The perthitic structure of the marginal zone of later feldspar is 

 slightly coarser (see Figure II, Plate 1). 



In irregular streaks and patches through the body of the crys- 

 tals the microperthitic structure is more distinct, and this coarsen- 

 ing is invariably accompanied by the occurrence of fine dust-like 

 particles characteristic of the potash member of the granite micro- 

 perthite. Sections of the homogeneous and cryptoperthitic parts of 

 the phenocrysts parallel to M, (010) have an extinction measured on 

 the 001 cleavage of from 12 to 13 degrees. Such sections show the 

 immergence of an obtuse bisectrix. Basal sections give a nearly 

 or quite parallel extinction. An acute positive bisectrix is obtained 

 from sections near 100 with an axial angle similar in size to that of 

 orthoclase. These properties are those of the cryptoperthite (an- 

 orthoclase) as described by Brogger, and of the feldspar pheno- 

 crysts described by Osann ^* in the Paisanite from West Texas. 



The smaller, and usually more irregularly shaped crystals lying in 

 the groundmass are for the most part microperthite (see Figure II, 

 Plate 1), this, arid the fact that the enlargements of the phenocrysts 

 are also microperthite, as well as the feldspar of the groundmass 

 proper, is important in connection with the relations of the crypto- 

 perthitic and microperthitic structure, as will be pointed out later. 



A striking feature of the phenocrysts is the extent to which they 

 are replaced by albite. Many crystals, particularly the longer ones, 

 are crossed transversely by narrow streaks of a more strongly polariz- 

 ing, colorless material (see Figure III, Plate 1). In it are often mi- 

 nute crystals of aegirite. This substance is evidently all)ite. It is 

 continuous with, and of the same orientation as the albite member of 

 the microperthite in the later marginal zone. These streaks represent 

 transverse fracture lines in the phenocryst along which albite set free 

 from the unmixing (see later Part II) of the original feldspar sub- 

 stance or from the crystallizing groundmass, or both, have entered. 

 Many instances are also seen where the albite has developed as a line 

 of minute crystals extending across the phenocryst, following the line 



34 Tschermaks Mtt. XV, p. 436. 



