12 THE ORIGIN AND ALTERATION OF ROCKS. 



this would not be a basis for assuming that nonnally sedimentary rocks take 

 these characters; although this statement is one which is frequently made. 



Examinations have been repeatedly made by the writer for the pui'pose 

 of ascertaining whether any rocks whose sedimentary origin was undoubted 

 had acquired the microscopical characters of eruptive forms, but nothing of 

 the kind has yet been discovered by him. 



In order to prove the passage of a sedimentary rock into an eruptive one, 

 it is necessary to have on one side the undisputed fragmental form, and to 

 trace it directly by continuous passage into the n on -fragmental one. Not an 

 inch of the parts lying between should be allowed to escape examination ; 

 and it must be positively known that no line of junction exists, but that the 

 two rocks form a continuous whole. In no case on record, however, does it 

 appear that passages of the kind indicated, and which have been claimed as 

 existing, have ever been subjected to so close an examination as is here 

 demanded. Eruptive and sedimentary rocks at their line of junction usually 

 mutually influence one another, often appearing very much alike, especially 

 when they have been subjected to later alterations by which both have been 

 affected. It is, then, to be expected that the observer who is not practically 

 f Miiiliar with these occurrences will pass directly over the lines of junction, 

 especially if he has been taught that direct passages of one rock into another 

 m:\y occur. His evidence is of that negative kind which, for various reasons, 

 can usually be obtained with ease. The evidence that the two rocks do not 

 pass into one another is of the positive kind, for the line of junction when 

 once seen can be examined and re-examined at any time ; while hand speci- 

 mens can frequently be procured which will show both kinds of rock and their 

 junction in one fragment. We can then have positive field and laboratory 

 (including microscopic) evidence that the two rocks are not the same but 

 different ones. The writer has had frequent occasion to examine localities 

 in which the direct passage of a fragmental rock into a non-fragmental one 

 was said to occur, and in no case has he not been able to obtain positive 

 evidence that such passage did not exist, when the conditions were such that 

 a satisfactor}^ examination could be made. When the evidence was lacking 

 it was alwaj^s owing to the junction being covered, or else shattered by 

 jointing, frost, etc. 



Practically, when the existence of these junctions had been shown, the 

 observers who had previously denied their existence have always said : 

 "That is not a typical locality; we were not quite sure about that place, 



