SUMMARY OP THE CONTENTS. XUC 



tories since 1828 ; a far-extending net-work of magnetic stations — ^p. 

 190 and note. Development of light at the magnetic poles; terrestrial 

 light as a consequence of the electro-magnetic activity of our planet. 

 Elevation of polar light. Whether magnetic storms are accompanied 

 by noise. Connection of polar light (an electro-magnetic development 

 of light) with the formation of cirrus clouds. Other examples of the 

 generation of terrestrial light — p. 202 and note. 



b. The vital activity of a planet manifested from within outward, the 

 principal source of geognostic phenomena. Connection between mere- 

 ly dynamic concussions or the upheaval of whole portions of the earth's 

 ci-ust, accompanied by the effusion of matter, and the generation of 

 gaseous and liquid fluids, of hot mud and fused earths, which solidify 

 into rocks. Volcanic action, in the most general conception of the idea, 

 is the reaction of the interior of a planet on its outer surface. Earth- 

 quakes. Extent of the circles of commotion and their gradual increase. 

 Whether there exists any connection between the changes in terres- 

 trial magnetism and the processes of the atmosphere. Noises, subter- 

 ranean thunder without any perceptible concussion. The rocks which 

 modify the propagation of the waves of concussion. Upheavals ; erup- 

 tion of water, hot steam, mud mofettes, smoke, and flame during an 

 earthquake — p. 202-218 and notes. 



c. Closer consideration of material products as a consequence of 

 internal planetary activity. There rise from the depths of the earth, 

 through fissures and cones of eruption, various gases, liquid fluids (pure 

 or acidulated), mud, and molten earths. Volcanoes are a species of 

 intermittent spring. Tempei*ature of thermal springs ; their constancy 

 and change. Depth of the foci — p. 219-224 and notes. Salses, mud 

 volcanoes. While fire-emitting mountains, being sources of molten 

 earths, produce volcanic rocks, spring water forms, by precipitation, 

 strata of limestone.. Continued generation of sedimentary rocks — p 

 228 and note. 



d. Diversity of volcanic elevations. Dome-like closed trachytic 

 mountains. Actual volcanoes which are formed from craters of eleva- 

 tion or among the detritus of their original structure. Permanent con- 

 nection of the interior of our earth with the atmosphere. Relation to 

 certain rocks. Influence of the relations of height on the frequency of 

 the eruptions. Height of the cone of cinders. Characteristics of those 

 volcanoes which rise above the snow-line. Columns of ashes and fire. 

 Volcanic storm during the eruption. Mineral composition of lavas — 

 p. 236 and notes. Distribution of volcanoes on the earth's surface ; 

 central and linear volcanoes ; insular and littoral volcanoes. Distance 

 of volcanoes from the sea-coast. Extinction of volcanic forces — p. 246 

 and notes. 



e. Relation of volcanoes to the character of rocks. Volcanic forces 

 form new rocks, and metamorphose the more ancient ones. The study 

 of these relations leads, by a double coui'se, to the mineral portion of 

 geognosy (the study of the textures and of the position of the earth's 

 strata), and to the configuration of continents and insular groups ele- 

 vated above the level of the sea (the study of the geographical form 

 and outlines of the diflerent parts of the earth). Classification of rocks 

 according to the scale of the phenomena of structure and metamorpho- 

 sis, which are still passing before our eyes. Rocks of eruption, sedi 

 mentary rocks, changed (metamorphosed) rocks, conglomerates — con* 

 pound rocks are definite associations of oryctognostically simple fossils 

 There are four phases in the formative condition: rocka of eruption. 



