KINDS OF ROCKS. 67 



albite and oligoclase, are called acidic feldspars ; and the others, es- 

 pecially vabradorite and anorthite, in which the feldspar is less than 55 

 per cent., basic feldspars. So also crystalline rocks, whether meta- 

 morphic or eruptive, are called acidic rocks when one of the chief con- 

 stituents is an acidic feldspar ; and basic, when it is a basic feldspar. 



Use of the Microscope in the Study of Rocks. — The stud}' of thin, transparent slices of 

 rocks by the microscope, is of interest whether the crystalline rock be coarse or fine in 

 texture ; but it is particularly important when of the latter kind. There is no rock so 

 opaque that it cannot be made transparent, or at least translucent, in thin slices. Such 

 slices are examined by means of a polariscope-microscope. The more important points 

 ascertained by this means, as regards the mineral constitution of the rock, are the fol- 

 lowing : — 



1. The presence or not of quartz-, of a feldspar ; of a chlorite. 



2. The distinction of a triclinic feldspar from orthoclase, the former showing in sec- 

 tions, cut in any direction excepting one, several parallel spectrum bands, due to mul- 

 tiple twinning in the crystal, while orthoclase shows no bands of the kind, or at the 

 most but two. 



3. The presence or not of hornblende ; this mineral having often cleavage lines 

 meeting at angles of 124°, and being dichroic. 



4. The presence or not of pyroxene ; this mineral often showing cleavage lines meet- 

 ing at angles of 87° (nearly a right angle), and being not dichroic, and usually dis- 

 tinguishable in this way from hornblende. 



5. The presence or not of mica, its cleavage lines meeting at angles of 60° and 120°. 



6. The presence or not of chrysolite; of magnetite, its form being often octahedral; of 

 points or portions having the nature of glass, and therefore not polarizing light; of 

 fluidal lines ; of globules of liquid carbonic acid, and of various other inclusions. 



For a particular account of the distinguishing characteristics of minerals brought out 

 by means of the microscope, reference must be made to treatises on the subject. 



The kinds of rocks are described under the heads of — 



1. Fragmental Rocks, exclusive of Limestones. 



2. Limestones, or Calcareous Rocks. 



3. Metamorphic Rocks, exclusive of Limestones. 



4. Eruptive Rocks. 



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. The names of the more common kinds of rocks 

 are printed in a bolder letter. 



1. Fragmental Rocks, exclusive of Limestones. 



1. Conglomerate. — A rock made up of pebbles or fragments of rocks of any kind. 

 (<i) If the pebbles are rounded, it is called pudding-stone ; (b) if angular, breccia. 



Conglomerates are named, according to their constituents, siliceous or quartzose, gran- 

 itic, calcareous, porphyritic, pumiceous, etc. 



2. Grit, Grit-rock — A hard, gritty rock, consisting of coarse sand, or sand and 

 small pebbles, called also millstone grit, because used sometimes for millstones. 



3. Sandstone. — A rock made from sand. There are siliceous, granitic, and micaceous 

 sandstones, according to the character of the material. There are also compact, friable, 



