MUSEUM OF COMPARATIVE ZOOLOGY. 171 



ger, " Die Sil. Etagen 2 uiid 3," etc., p. 2G1). In the rock as a whole 

 the same even balance between ISTa and K is noticeable, and the insig- 

 nificant quantity of lime and magnesia. Allowing for the free quartz, 

 base, and decomposition products as causing a relative increase of silica 

 and iron and decrease of the alumina and alkalies, it is evident that the 

 feldspars of the gi'oundmass are closely allied chemically to the porphy- 

 ritic crystals, and ai'e probably also anorthoclasc. The rock is therefore 

 a very pure type of keratophyre. 



The microscopical structure of the sections made are as follows : — 



No. 21. Keratophyre witli anorthoclase crystal cut obliquely to an optic 

 axis. Groundniass made up of minute twinned lath-sbaped crystals of feld- 

 spar, somewhat kaolinized, some quartz, and an earthy fibrous kaolinized base. 

 In the centre of the porpbyritic feldspar crystal are numerous microliths and 

 a few ferritic masses, similar to and probably composed of the base, which 

 penetrates the edges of the crystal. 



No. 21 A. Keratophyre and an aggregate of the porpbyritic crystals. 

 Groundniass nearly as in No. 21. One of the pheuocrysts shows twinning 

 after the Carlsbad type. 



No. 21 B. Keratophyre with one porphyritically enclosed crystal. The 

 crystal is cut nearly parallel to the second cleavage, and gives an almost per- 

 fect interference figure of the positive bisectrix. The basal cleavage is well 

 developed, and the stria), or fine twinning, are well marked in polarized light 

 The groundmass is more generally composed of the minute lath-shaped feld- 

 spar crystals, some of which are clearly twinned anorthoclase of the same form 

 as the larger crystals. There are also small patches of quartz. 



No. 21 C. Keratophyre with one large porpbyritic feldspar crystal cut 

 obliquely to the brachydiagonal, which in polarized light shows a niicroper- 

 thitic intergrowth and a very perfect example of fine and interruiited twinning. 

 Through the crystal are several fluid cavities and a few microliths of a reddish 

 color. The groundniass is more kaolinized, and the minute lath-shaped crys- 

 tals are less distinct. Small irregular masses of quartz and considerable 

 limonite and earthy matter pervade this section. 



No. 21 D. Keratophyre section cut across a joint plane which is filled with 

 vein quartz ; numerous irregular patches of quartz are scattered all through 

 the section. On^mass is a basal section of original (?) quartz ; it gives the 

 uniaxial cross, and is shown to be positive by the mica plate. Some scales, 

 of biotite and numerous small grains of magnetite are seen in the groundmass, 

 which is composed of a fibrous feebly polarizing kaolinized mass of the decom- 

 posed minute lath-shaped feldspar crystals. One of the enclosed pheuocrysts 

 cut nearly parallel to the base shows nuniejous inicrolithic inclusions, and sev- 

 eral fluid cavities in which the Ijulible muvemeut i; seen The outer edge is 

 deeply penetrated l)y the LCioundiiuiss 



