SYNOPSIS OF author's ARGUMENT. 261 



Scope of the Paper. 



Synopsis of Argument. — The argument of this paper is briefly as follows : 



a. The earth in its early state absorbed various gases which now it 

 tends to give off, having solidified in process of cooling. These gases are 

 an original and essential factor in every igneous magma, and their reten- 

 tion is necessary to the crystalline development and texture of the 

 plutonic and dike rocks, as is their sudden loss to the development of 

 volcanic rocks. 



b. Whenever cracks extend down into the solid earth far enough these 

 gases escape and are the essential moving cause of volcanic eruptions by 

 carrying with them the lava, just as champagne is carried out of a bottle 

 by the imprisoned gas. 



c. The earth after getting below the various surface formations gradu- 

 ally increases in basicity, weight and heat toward a core mainly iron. 



d. Thus lavas first delivered from a crack will be a mixture from 

 different levels of the earth. Their chemical* composition will vary 

 according to the depth of the crack and other casual circumstances, but 

 will be of medium basicity as regards the material that can be furnished 

 from the crack. Any already individualized minerals of pronounced 

 acid or basic character, as, for example, quartz or olivine, will be corroded 

 by the resultant magma. 



e. Acid Javas would be furnished particularly by shallow cracks. They 

 would have a. less high temperature than the basic and be in a pasty 

 condition. The deeper seated basic lavas would have a higher tempera- 

 ture and hence take a longer time to cool, and hence give their vapors 

 more time to escape before final consolidation. 



/. In plutonic rocks these primal absorbed gases, slowly given off after 

 intrusion, produce in the adjacent rocks the characteristic contact-zone. 

 In volcanic rocks the gases escape from the vent, and hence the contact- 

 zone is of different character, if at all present. 



(J. These gases may also be concentrated in the crystallization of 

 plutonic masses (batholites) into the pegmatite and other dikes, and 

 assist in giving them their i)eculiar character, and there is a continuous 

 series from pegmatites to segregation veins and true fissure veins filled 

 by ascent, so that there is justification for classing the pegmatites with 

 either veins or igneous rocks. The gases in such cases are doubtless 

 condensed into a strongly mineralized hot water. 



h. If the gases percolate outward in all directions with extreme slow- 

 ness, we should expect them to tend to produce a regular gradation in 

 composition from the center outward and to cause an exchange of ma- 

 terial in alternating beds of different composition. 



