10 THE ORIGIN AND ALTERATION OF ROCKS. 



semi-fluidal, or fluidal state. All these changes should exist in the same 

 continuous mass of rock, and we ought to be able to trace the gradations 

 from one place to another. That such passages have been observed, has 

 been repeatedly claimed, but when the localities where these facts could 

 be observed were sought for, they could not be found. 



The results of petrographical study seem to point to the following as the 

 probable origin of rocks. If we start from a cooling liquid earth then all 

 mechanically and chemically formed rocks have come from the liquid ma- 

 terial originally. Furthermore, all the eruptive rocks appear to have come 

 from below the sedimentary ones, and are onl^^ influenced by them in their 

 composition, by the materials accidentally picked up during their passage 

 through, or flow over the latter. In the case of volcanic rocks, we should 

 expect to have associated with the lava, ashes, and in fact every kind of 

 material projected from the crater, including debris and mud.- All these 

 would be naturally more or less intimately mixed together according as one 

 was deposited on, or around the other, — or as one in its flow picked up, sur- 

 rounded, or overlaid another. This would associate all loose materials and 

 rocks of any kind that existed in the locality prior to the lava flow; while 

 during that time and later the atmospheric agencies would tend to still more 

 intimately mingle these diverse materials, and obliterate their differences. 

 Wherever the lava was exposed to detrital action, there would be deposited 

 about and around it detritus of the same material, mixed or not, as the case 

 might be, with that from other rocks, — especially if the eruption took place 

 on or near the shore line. In the case of massive or fissure eruptions and 

 dikes, we should expect but few or none of the common accompaniments of 

 an ordinary explosive volcanic eruption, but all eruptive material would be 

 subject to degradation, and would under proper conditions become associated 

 with its own detritus and that formed from other rocks. All the associated 

 detritus, if of one kind, would suffer the same alterations which non-frag- 

 mental material of the same kind has to pass through. Under conditions 

 otherwise identical, detrital material would doubtless be affected in a grenter 

 degree than the solid rock, owing to the former's greater perviousness to 

 water. While in tlie unaltered condition we may be able to readily distin- 

 guish the fragmental from the non-fragmental forms by the unaided eye, this 

 is no longer possible when both have been subject to alteration. They then 

 closely simulate one another, and the microscope used in connection with 

 the field evidence offers the only means of distinguishing the fragmental 



