150 Naming Rocks. 
Crystalline is a term employed to express the struc- 
ture of rocks. Many rocks which appear to the unaided 
eye to be compact or massive, under the microscope 
often exhibit complete crystalline structure. 
Glassy, or vitreous, are terms used to express the 
presence of glass which occurs in the bases of most all 
igneous rocks. 
Porphyritic is the term employed to denote a struc- 
ture in which a few crystals have separated themselves 
out and crystallized apart from the ground mass of the 
rock; a rock is said to be porphyritic especially when 
these crystals consist of feldspar. 
Amygdaloidal rocks are those in which the almond- 
shaped vesicles have been filled up by other minerals 
brought in, probably by percolating water, such min- 
erals are termed “zeolites.” 
Vesicular rocks are those containing spherical cay- 
aties. 
Fluidal is a term applied to rocks of a wavy or 
streaky appearance ; it is common in rhyolyte, and marks 
the movement of particles in a once molten rock. © 
Schistose is the name given to a structure which 
has been induced in rocks by metamorphism. The struc- 
ture consists in the arrangement in wavy, irregular lines 
and layers of the different minerals. 
Oolitic-Pisolitic—Oolitic is the term used to ex- 
plain the structure of a rock which is like the compact 
roe of a fish. When the grains are large the rock is 
called “pisolitic.” 
UNSTRATIFIED, METAMORPHIC AND ERUPTIVE ROCKS. 
Granite—Composition, quartz, orthoclase and mica, 
metamorphic and eruptive. The chemical composition 
of granite agrees very nearly with that of some sand- 
Bale ie. B 
