



ALTERATION OF ROCKS. 



97 



i 





fi 



f 



Wall-case 45. 



Arranged and described by A. C. Ramsay. 



Specimens to Illustrate the Phenomena of Alter- 

 ation AND MeTAMORPHISM OF ROCKS. 



The subject of the alteration of rocks is so large, that, 

 to enter upon it fully, would require greater space for 

 specimens than the Museum affords. In Cases 45 and 

 46, some of the more striking phenomena are illustrated, 

 1st. the effect of intense heat on various hinds of rochs ; 

 2nd. the more remarkable process of metamorphism by 

 tohich new mineral combinations are developed, as in mica 

 slate and all the rochs of the gneis sic family. In CaSG 46, 

 the phenomena of cleavage are illustrated, which in some 

 instances bears a peculiar relation to metamorphism. 



Nos. 1 to 75 have special reference to the first, or more 



*/ 



i of com/ 



The list 



effects off 



begins with 



alterations of the most 



obvious character, such as those produced on coal by the 

 intrusion of trap dikes, on common slate by the burning 

 of a house, on sandstone in an iron furnace, and from 

 this the student is readily led to those alterations which 

 from common sandstones produced quartz rochs by the 



them of melted matters, which, when 



among 



intrusion 



cooled and consolidated, became basalts, greenstones, syen- 

 ites and quartz porphyries. Of the most intense altera- 

 tions of this kind, Nos. 1 to 46 give a striking example, 

 showing a passage by alteration from slate and grit into 

 actual syenitic greenstone, for some of the grits and con- 

 glomerates have been on the very verge of fusion, if not 

 undoubtedly fused, and though pebbles and some of the 



nular character may (especially on the ground) 



detected 





the whole having become 



Upper 

 Gallebt. 



Wall-case 45. 



