vi CONTENTS. 



origin of rocks deraamlcil by the results of pctrograpliical study, 10. Tho asso- 

 ciations of difFereut rocks, and the difficulties in their separation, 10, 11. The 

 microscopic and field evidence the only means of distinction, 10, 11. Eruptive and 

 Sedimentary llocks resemble each other through tlic alteration of both, 11. Sedi- 

 mentary Rocks presenting peculiar and abnormal conditions not proper guides, 11, 



12. Sedimentary Rocks found not to present the microscopic characters of Erup- 

 tive ones, 12. Eequirements necessary to prove the passage of a Sedimentary 

 into an Eruptive Rock, 12. Generally positive evidence that such passage does 

 not exist can be obtained, 12, 13. The chemical resemblance of Sedimentary 

 and Eruptive Rocks owing to the derivation of the former from the latter, 13. 

 Minerals in Lavas of prior origin to the consolidation of the magma, 1.3 ; not de- 

 rived from Sedimentary Rocks and characteristic of ancient and modern Eruptives, 



13, 14. Field and micro.scopic evidence opposed to the theory of the derivation 

 of Eruptive from Sedimentary Rocks, 14; the demands of that theory, 14. Vol- 

 canic or eruptive action began in the earliest ages of the Earth, 14 ; this action, 

 although intermittent, is a dying one, 14, 15. The older Eruptives the same, origi- 

 nally, as the modern ones, 1.5 ; their present differences due to alteration, etc., 15. 

 Under like conditions alteration is proportional to the age, 1-5. The presence of 

 fragments of one rock in another is alone no proof of difierence in geological ago, 

 15. The alteration produced by internal molecular or chemical changes in the 

 rock mass not to be confounded with superficial weathering, 15. Metamorphism 

 not extended Pseudomorphism, 15. Pseudomorphism bat an incidental phase in 

 alteration, 15. The explanation of changes in rocks, 15, 10. Metamorjihism 

 inversely proportional to the contained silica in the original rock, when time 

 and other conditions are the same, 16. Eruptive Action, including Thermal 

 Waters, an efficient agent in Metamorphism, 16. Metamorphism dependent on 

 the chemical composition of the rock and the metamorphic agents, hence litlio- 

 logical characters no criterion for determining geological age, 16. The constitu- 

 ents of rocks pass from an unstable towards a more stable condition, 16; this 

 passage a factor in the dissipation of energy, 16. Rocks are produced, grow old, 

 and decay, but are not raised again, 17. Crystalline Structure no proof of great 

 age or of great depth, 17. Longtime not always allowed for the formation of 

 fine-grained and fossiliferous rocks, 17. Coiitracti(m tends to maintain a uniforaa 

 temperature in tho Eartli's interior, 17, 18. Relative Progression in geological 

 time from aliundant Acidic to abundant Basic Eruptives, 18. All Eruptives de- 

 rived from the Earth's interior material which had never solidified, or which has 

 since been reliquefied, 18. Sokbv's method of determining the origin of a rock 

 misleading, 18. Association of Rocks is alone no proof of community of origin, 



18, 19. Crystalline Schists naturally occur in a region of eruptive rocks, 19. 

 Definition of Lamination, 19 ; this structure common in many eruptive rocks, 19. 

 Joint Planes defined, 19; often mistaken in eruptive rocks for Bedding Planes, 



19, 20. Cleavage defined, 20. Cleavage conmion both in Eruptive and Sedi- 

 mentary Rocks, 20. Foliation defined, 20. Foliated Limestone mistaken for 

 Mica Schist, 20. Foliation and Cleavage produced by the same cause at Squantum, 

 Mass., 20, 21. Foliation common in altered eruptive ru£ks, 21 ; the planes being 

 at right angles to the direction of pressure, 21. Schistose or Fissile Structure, 21. 

 Fluidal Structure defined, 21. Schistose Structure often mistaken for Fluidal 

 Structure, 21 ; the latter mistaken for Planes of Sedimentation, 21. Lines of 

 chemical deposition taken for fluidal structure, 21. Arguments from analogy of 

 doubtful value, 22 ; also from one region to another, 22. Evidence not sustain- 

 ing the divisions of the Azoic System, 22, 23. Explanation of the structure of 



