Wadsworth.] 198 [May 7, 



are hereafter to exist. For a time, at least, prior to their erup- 

 tion they have been placed in far different conditions from the 

 atmospheric ones on the earth's surface ; and, of necessity, there 

 will be a constant tendency on their part to conform to these 

 changed conditions. This is manifested most conspicuously in 

 their loss of heat, and their passage from a liquid to a solid con- 

 dition. When solid, it may be said that these rocks are in an un- 

 stable condition, in respect to their temperature, and also to the 

 chemical combinations formed on solidification. Their chemical 

 arrangements, as manifested in their constituent glass and min- 

 erals, are such as to necessitate a transference to a condition in 

 which they are less acted upon by the agencies to which they are 

 exposed, on the earth's surface and this leads to a degradation, 

 dissipation, and loss of energy on their part. In other words, 

 the rocks tend to pass from an unstable towards a more stable 

 state. The rapidity of these changes depends not only 

 upon their chemical constitution, but also upon the special 

 circumstances in which the rocks are placed. In the basic rocks, 

 or those containing much iron, magnesium, calcium, aluminum, 

 etc., the alterations are comparatively rapid, but in the acidic rocks 

 much slower. If rocks of eruptive origin are studied under the 

 microscope these alterations can be readily traced from their 

 beginnings to the extreme changes, which are usually found to be 

 proportional to their age or some special condition. 



It is these alterations which have led to the multiplicity of 

 rock names, and to the confusion of nomenclature ; lithologists and 

 geologists generally proceeding on the supposition that as a rock 

 now is, so it always was and always will be. For example, the lava 

 flows of Keweenaw Point, which were once identical with the modern 

 basaltic lavas of Mt. Etna and Kilauea, are now designated, on 

 account of their alteration and age, as melaphyrs, diabases, 

 diorites, etc. ; andesite in its changed guise is designated as 

 propylite, diabase porphyrite, porphyrite, diorite, etc. ; rhyolite 

 as felsite, quartz porphyry, petrosilex, orthofelsite, etc. ; peridotites 

 or olivine rocks, as serpentine, talc schist, etc. 



The propylite of the Comstock Lode is a striking example. 

 The present writer was the first to call attention to the fact 

 that the fortieth parallel propylites were altered forms of 



