KXAMLNATIOX OF CUIU'JvN'T TIIKOHIKS. 11 



from the non-fragnientjil (orm. When rocks of more than one kind are 

 mixed in the detritus, tlie alteration and aj)|)earance ol'the .sedimentary rock 

 formed from this undergoes a corresponding modification. 



We .should expect to find certain very intimate relations hetween all 

 these various forms of associated rock, and it would be very dilhcult to di.«- 

 tinguish, in the older and more altered forms, between the material picked 

 up during the flow, the ashes or debris, and the solid non-fragmental rock. 

 The greater the amount of secondary alteration which these different rocks 

 have suffered, the greater the difficulty of distinguishing between them. In 

 no case, however, would the frngmental pass into the non-fragmental form 

 by insensible gradations or otherwise. It is true that they sometimes appear 

 to do so, but that appearance is only superficial. 



In order then to decide between the different theories proposed for the 

 origin of eruptive rocks, it is necessary to make some examination of the 

 evidence offered in their support by petrographical study. For this pur- 

 pose the most important question is, do sedimentary rocks take upon them- 

 selves the characters of eruptive ones ? In the writer's studies he has found 

 a certain resemblance between both classes of rocks when they are of similar 

 composition. This is, however, only in the case of rocks greatly altered, and 

 arises from secondary changes in each ; which result in the production of 

 new mineral constituents, and in the obliteration of the original structure 

 of both to a greater or less extent. Indeed, in some cases, this obliteration 

 is total, the minerals and mineral characters — in fact all the characters — 

 of the rocks thus changed being rendered unlike those which belonged to 

 the original eruptive rock. These alterations are apparently- more depend- 

 ent upon the chemical composition of the rocks, and the conditions to which 

 they have been subjected, than upon their having been in a fragmental or 

 non-fragmental state. The result is, that the eruptive rock is degraded to 

 the status of an altered sedimentary rock, not that the latter take* upon 

 itself the characters of an eruptive one. W^hether the two classes thus 

 indicated can or cannot always be distinguished under the microscope in 

 cases of extreme alteration, is a problem of the future, and perhaps the 

 most difficult one with which the petrographer will be confronted. 



Undoubtedly, a careful study of the field relations of rocks would, in the 

 majority of cases, suffice to settle the question of their origin. 



If sedimentary rocks should be found under peculiar and abnormal con- 

 ditions, to present the characters regarded as typical of eruptive ibrms* 



* Metamorpliism produced by the l)iiniiiig of Tjigiiite Beds in Dakota and Montana Torritories. Bv J. A 

 Allen. Pioc. Bost. Soc. -Nat. Hist., 1S74, xvi. 24(J-3(5-2. 



