308 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



Instances of regional metamorphism are not wanting. Sandstones are 

 changed to quartzites, dolomites frequently into marbles and less often more 

 or less serpentinized; but these changes cannot be assigned to the direct 

 action of heat, since they are in no sense contact phenomena. Their devel- 

 opment is local and irregular, extending over considerable areas, where 

 there is no actual contact of the altered beds with intrusive rocks, and, on 

 the other hand, being more generally absent from the actual contact with 

 these rocks. 



Non-absorption of sedimentary rocks by erup'.ive masses. Another important obser- 



vation in regard to these intrusive bodies, and in one sense a corollary of the 

 above statements, is the fact that, although they have split apart and pried 

 open the sedimentary strata and caught up or entirely surrounded both 

 large and small fragments of sedimentary rocks, there is no evidence of 

 their having absorbed or assimilated within themselves by actual fusion 

 any portion of these sedimentary rocks; certainly not any considerable 

 masses thereof. Not only are there no relics of fusion at the present con- 

 tact, as there necessarily would have been if a portion had already been 

 fused, but in reconstructing the sections on actually measured profiles there 

 is no portion of the sedimentary strata missing, which cannot be accounted 

 for by erosion. Along the contact surface the fused mass has cracked off 

 fragments, often quite small, which have consolidated again into a sort of 

 breccia ; again, the thinner sheets have sometimes Itent back and contorted 

 a stratum of limestone or quartzite at the end of the flow or as it crossed 

 from one bed to another; but of fusion, as already stated, there is no sign. 

 I have insisted on this point because the question of the capability of 

 an igneous mass to absorb, or eat up as it were, the sedimentary or even 

 already consolidated igneous rock through which it passes, is one which 

 has always interested me, and for which, in a field experience of over fifteen 

 years, largely among eruptive rocks, I have vainly sought for demonstrable 

 proof. It is customary among geologists to draw their ideal underground 

 sections of igneous masses as if this capability were unlimited, and geo- 

 logical text-books seem to tacitly assume that it is so, without offering an 

 explanation of how it is possible or the grounds on which the assumption 

 is made. 



