METHOD OF REASONING. 37 



beyond the zone of observation, accomplishing- at the present time similar 

 results. This is a complete reversal of the Huttonian method. Hence, in 

 treating of metamorphism we must argue both from the present to the past 

 and from the past to the present. By studying the action of the forces and 

 agents now at work in the zone of observation and the stages of alteration 

 preserved in the rocks brought into the zone of observation we are able to 

 push the boundaries of the known for a certain distance into the domain of 

 the unknown, and infer with considerable certainty the nature of the 

 changes which have taken place in the far-distant past and of those which 

 are now taking place but which we can not directly observe. 



It will be generally agreed that the majority of the altered rocks, 

 including' a large portion of the schists and gneisses, have been metamor- 

 phosed from aqueous and igneous rocks like those now being produced. 

 This is in accordance with the Huttonian principle. But some may hold 

 that the most ancient of the schists and gneisses, those of the so-called 

 Basement Complex, had a different origin. For instance, it has been held 

 by some that these ancient rocks are direct precipitates in a primeval 

 ocean. On later pages it will be seen that the most ancient schists and 

 gneisses are in all respects like those produced from more recent rocks by 

 the processes of alteration, and therefore that the probable, but not certain, 

 inference is that they were produced from rocks not fundamentally unlike 

 those now being formed by processes of change not radically different from 

 those now at work. But in ascertaining the forces and agents and their 

 method of work in both the ancient and the modern rocks, we must for the 

 most part follow the reversal of the Huttonian principle — i. e., argue from 

 past results as to the nature and method of work of present forces and 

 agents. 



Whatever the origin of rocks — whether solidifications from magmas, 

 chemical precipitates, organic deposits, or mechanical deposits — as already 

 noted, they may be altered so as to modify their structures, so as to change 

 their mineral composition, and so as to change their chemical composition. 

 In place of the original characteristic structures of the igneous rocks, such 

 as flowage structure and massive structure, and in place of the original 

 structures of the sedimentary rocks, such as bedding, there may. be pro- 

 duced secondary structures, such as cleavage, fissility, joints, slatiness, 

 schistosity, and g-neissosity. In place of the original textures of igneous 

 rocks, such as granolitic, porphyritic, ophitic, and poikilitic, and in place 



