MICROSCOPICAL INVESTIGATION. 551 
If a microcrystalline structure prevails, the feldspar is formed in 
narrow, long and relatively large prisms, while the quartz recedes 
considerably and appears at the margins as if blended with the 
remaining part of the ground-mass. This also agrees with the small 
amount of silica of such varieties, from which we may infer that only 
about 15.20 % of quartzis present. Though the feldspar crystals are, 
it is true, rendered more or less opaque, still numerous twin striations 
can often be recognized. Glass and microfelsite in Rosenbach’s sense 
could not be recognized with certainty. 
Besides quartz and feldspar, biotite occurs irregularly distributed 
through the ground-mass in the form of small lamella which decrease 
in size to the smallest microliths; their number varies but is always 
very small, and the biotite being mostly much altered, it is not always 
easy to decide whether brownish particles represent decomposed mica 
or little films of hydrated oxide of iron. In one of the liparites the 
biotite seems to be replaced by muscovite. At least J think it per- 
missible to explain in this manner the numerous colourless little 
staves and lamelle shewing vivid interference colours, although no 
accurate determination was possible. They completely resemble the 
lamelle which are so often to be met with in the ground-mass of 
quartz-porphyry, and are generally considered to be muscovite. 
Opaque ores and zirkon appear only occasionally and in exceedingly 
small quantity. 
Of interpositions, especially quartz and feldspar (which are also 
macrospically recognizable) occur chiefly, and their number only 
increases slightly under the microscope. Occasionally some biotite is 
present in addition. Once a large grain of a regular colourless 
mineral was observed, which judging from its strikmg appearance 
and its somewhat scaly surface when ground may probably be 
fluorspar. 
The quartz contains colourless microliths, pellucid glass with one 
or several bubbles, glass with products of devitrification, and ground- 
mass, but the number of enclosures is always small. Together with 
regularly defined quartz there very frequently appear some which are 
quite irregularly formed and sharply angular, and which are no 
doubt fragments of larger fractured grains. In the liparites of 
Gebbie’s Pass, the feldspar is fresh, glassy, mostly composed of 
sanidine, and often full of peculiarly formed gas-pores. In the other 
varicties the feldspars are much altered, and indeed as it appears 
into a pinitoid-like substance. Zirkon appears quite isolated as 
enclosures, and could be determined by its crystalline form, strong 
refraction, vivid interference colours and parallel extinction with 
sufficient certainty. The biotite which appears especially in the 
variety of Gebbie’s Pass rich in interpositions is generally very much 
altered into a brown, somewhat translucent substance, which points 
to a mica rich in iron. 
The fine veins which traverse the ground-mass of several liparites 
are formed of quartz and feldspar accompanied by hydrated oxide of 
iron, which as it seems has originated by decomposition of the biotite. 
The rare occurrence of plagioclase and basic components is charac- 
teristic of many liparites, but is especially prominent here. 
