CONTENTS. vii 



districts of Crystalliue Rocks, 22. Application of current views in American 

 Geological Literature, 23. Association of Eruptive and Volcanic Rocks, 23. 

 Origin of Crystalline Rocks, 23. Application of current views to Vesuvius in 

 the time of Str^ujo, 24. Principles to be emploj-ed in studying regions of 

 Crystalline Rocks, 24. Jlaterials of the earliest formed Lands of eruptive origin, 

 24. Application of the terna Eruptive or Volcanic in this work, 24. The younger 

 Volcanic and the older Plutonic Rocks form a continuous series, 24. 



SECTION in. 



The ORicm and Eelatioxs of the Mineral Constituents of Eocks . . . 25-30 



The constituents of rocks fill into three classes, 2.5. Two divisions of the first 

 class, 25 ; action of the Magma on Minerals of the first division, 25. Inclusions 

 in eruptive rocks, 25, 26. Action of Lava on Inclusions, 20. Microscopic charac- 

 ters of Eruptive Rocks opposed to the theory of their derivation from Sediraent.s, 

 2C. Jlinei'al products of the crystallization of a magma, 2(5. Mineral products 

 of rock alteration, 20. Tiic chemical constitution of altered rocks not essentially 

 changed, 26. Cause of rock alteration, 27. Alteration a character of the rock 

 mass as a whole, 27. Altered eruptive rocks tend to simulate the features of 

 sedimentary forms, 27. The general tendency of rock alteration, 27, 28. The 

 concentration of ores in rocks and veins attendant upon rock alteration, 28. Ores 

 of mechanical and eruptive origin excepted, 28. Theorj' of ore deposits, 28. 

 Mineralogy and Economic Geology chiefly sciences of abnormal minerals, 28. 

 Unstable character of eruptive rocks, 28 ; their resemblance to chemical labora- 

 tories, 29 ; passage from unstable towards more stable chemical combinations, 29. 

 Induration not alwaj's an indication of exposure to heat, 29. The glassy state of 

 rocks is nearest their primitive condition, 29. Designations employed for the 

 three classes of rock-forming minerals, 29. Distinction of cases of envelopment 

 from alteration products, 29, 30. Application of the principles of Thermo-optics, 

 30. The pyroguostic characters of a mineral have little or nothing to do with its 

 condition before its formation, 30. The conditions under which minerals crystal- 

 lize from a cooling magma are different from those under which vein and altera- 

 tion minerals are formed, 30. 



SECTION IV. 

 CiiEMic.vL Analysis of Eocks 31, 32 



Chemical Analysis unable to determine the mineral constituents of rocks, 31, 

 32. While Chemical Composition remains nearly constant, great variation exists 

 in the structure and mineral constituents, 31. What Chemical Analysis can do 

 for the lithologist, 31. Relation of Chemical Analysis of normal rocks to rock 

 species, 32. Analyses should be written in terms of the elements, instead of their 

 compounds, 32. 



SECTION V. 



Classification based on Mineral Composition 33-45 



Basis of the common classifications of rocks, 33 ; minerals used, and data re- 

 quired, 33. — The Feldspars, 33-43. Their different modes of origin, 33. 

 Scheeker's feldspar theory, 33, 34; Delesse's views, 34 ; Hermann's molecular 



