78 STRUCTURAL GEOLOGY. 



8. Vesicular. — Having small cavities in the rock (igneous), made usually 

 by steam, as in many lavas. 



9. Scoriaceous. — Having vesicles in so great abundance that they make 

 the chief part of the mass like much furnace slag, as a scoriaceous lava. 



10. Amygdaloidal (from amygdalum, an almond). — Having the vesicles 

 (which are often almond-shaped) filled with minerals foreign to the rock, 

 such as quartz, calcite, and the zeolites. Trap, or doleryte, and related basic 

 eruptive rocks are often amygdaloidal. 



The following are other terms used in describing either fragmental or crystalline 

 rocks : — 



Quartzose. — Consisting of quartz ; containing much quartz. 

 Calcareous. — Consisting of limestone (calcite) ; containing much calcite. 

 Ferruginous. — Containing much iron oxide. 



Argillaceous (from argilla, clay). — Made of more or less hardened clay or fine mud ; 

 containing clayey material. 



Pijritiferous. — Contaming pyrite. 

 Cfranitic. — Made of granite sand, or gravel. 



(2) Descriptions of rocks. — The kinds of rocks are described under the 

 heads of — 



Limestones, or Calcareous Rocks. 

 Fragmextal Rocks, kot Calcareous. 

 Crystalline Rocks, exclusive of Limestones. 



In the names of rocks, the termination ite is here changed to yte, as done in the 

 author's System of Mineralogy (1868), in order to distinguish them from the names 

 of minerals. Granite is excepted. 



Limestones, not Crystalline. 



Massive Limestone. — Compact uncrystalline ; color whitish, dull gray, bluish gray, 

 brownish, and black. Texture compact to earthy, sometimes semi-crystalline. Consists 

 essentially of calcite or calcium carbonate (page 68), but is often impure with clay or 

 sand. 



Most limestones are of organic origin. A dark or black color is usually owing to 

 some carbonaceous material present, derived from the decomposition of the plants or 

 animals of the waters in which they were formed. When burnt, limestone (CaOgC) becomes 

 quicklime (CaO), through loss of carbonic acid (COj) ; and, at the same time, all 

 carbonaceous materials are burnt out, and the color, when it is owing solely to these, 

 becomes white. A limestone made of pebbles of limestone is called a limestone conglom- 

 erate, as that of the Potomac. 



Magnesian Limestone, Dolomtte (page 68). — Calcium-magnesium carbonate. Not 

 distinguishable in color or texture from ordinary limestone. Much of the common lime- 

 stone of the United States is magnesian. While some of the magnesian limestone is true 

 dolomite (or has the calcium and magnesium in the atomic proportion 1:1), much is a 

 mixture of calcite and dolomite. 



In some limestones the fossils are magnesian, while the rock is common limestone. 

 Thus, an orthoceras in the Trenton limestone of Bytown, Canada (which is not mag- 

 nesian), afforded T. S. Hunt calcium carbonate 56-00, magnesium carbonate 37*80, iron 



