73 LITHOLOGICAL GEOLOGY. 



Silurian limestone of Bernardston, Mass., and Devonian strata full of fossils at Moose 

 River, in Nova Scotia, contain beds of magnetic iron ore, and at Nictaux there is in 

 the Devonian a bed of hematite six feet thick. Titanic iron occurs in great beds of 

 like extent in Canada, and is mixed with the magnetite of northern New York and 

 western North Carolina. Itabyrite is a mica schist consisting largely of hematite. 



Franklinite, an iron-zinc ore, is also one of the metamorphic rocks in northern New 

 Jersey. 



4. Igneous or Eruptive Rocks. 



Igneous rocks are those which have been ejected in a melted state, either from vol- 

 canoes or through fissures in the earth's crust. Their most general characteristics are: 

 (1) the presence of a feldspar as one of their constituents; (2) with most kinds, ab- 

 sence of free quartz; (3) occurring often as the rilling of fissures (pp. 738, 740), as 

 well as in overlying masses, or intercalated between layers of stratified rocks. 



Igneous rocks are not always easily distinguished from metamorphic rocks, or those 

 of the veins, and a few kinds of the two divisions are identical. In the nietamoj-phic 

 process, a stratified rock has sometimes been reduced to a pasty state, and in this con- 

 dition has been forced into fissures, and so has taken the position, and, as it cooled, 

 the ciystalline texture and aspect of an igneous rock. Some granite is an example. 

 Again, true igneous rocks have at times resulted from the fusion (or an equivalent 

 softening) of preexisting crystalline rocks (granite, S3 r enyte, and the like), and so have 

 derived a constitution more or less resembling that of the rock out of which they were 

 made. Thus igneous rocks, although generally containing little or no quartz, may in 

 some cases abound in grains of this mineral. 



The same igneous rock often occurs in various conditions, dependent (A) on rate of 

 cooling; and (B) on its remaining unaltered, or its being altered by water and other 

 vapors received from sources in or among the earth's strata when the melted rock was 

 on its way to the surface. It may thus vary — 



A. From coarsely crystalline granular to aphanitic; from an even-grained rock to 

 coarsely porphyritic, in which the feldspar is in distinct crystals; from the condition in 

 which every grain, as seen in thin slices with the microscope, is a defined mineral, to 

 that in which portions or points are in the state of glass or " unindividualized," from 

 an even homogeneous texture to one in which there are fluidal lines, as explained 

 on p. 65. 



B. From the condition in which the minerals of the rock, feldspar, hornblende, 

 and augite become pellucid when the rock is sliced thin, to that in which they are 

 more or less clouded, and a green chlorite — a hydrous species — occupies the place 

 of part of these minerals, and perhaps other results of alteration are manifest ; also 

 from the condition of perfect compactness to that of a cellular or amygdaloidal struc- 

 ture, amygdaloidal cavities being a result of vapors. 



Hence criterions for distinguishing kinds of igneous rocks based on their being 

 coarse-grained or aphanitic; not porphyritic or porphyritic : containing glassy grains in 

 the mass, or being wholly " individualized ; " showing fluidal lines or not ; being unal- 

 tered and anhydrous, or, on the other hand, hydrous, chloritic, and amygdaloidal, are 

 unsatisfactory. 



Further, rocks, as objects in science, should be defined and named according to their 

 hinds, — not according to the era of formation, — since the same things are the same 

 whenever made. The distinction of diabase from dolervte, based on being pre-Tertiary 

 or not, and others similar, has been proved to be bad lithologically as well as geologi- 

 cally. 



Apatite in microscopic crystals, and magnetite or titanic iron in crystalline grains, 

 are present in nearly all varieties of igneous rocks. 



Eruptive rocks may be arranged in three series, though no strong lines can be drawn 

 between them. 



1. Afeldspathic series, the species containing little or no hornblende or pyroxene, 

 and hence but little iron, and of low specific gravity (2-4-2'7). 



