1076 THE MICEOSCOPE IN GEOLOGICAL INVESTIGATION 



salt slowly evaporating. Restoration of the broken angles first takes 

 place ; then deposition goes 011 over the whole exposed surface, in 

 perfect optical and crystalline continuity, so as to change a broken 

 fragment into a definite crystal. A similar process frequently takes 

 place in limestones which are not absolutely pure. 1 Sometimes this 

 secondary deposit is carried so far on the grains of a clean sandstone 

 that the interstices are completely filled up and the rock is converted 

 into a quartzite. 



By the microscopical examination of volcanic dust or ashes it is 

 possible to determine the constitution of the igneous mass whose 

 eruption gave rise to such material. Thus the ashes and dust which 

 fell at various places after the great Krakatoa eruption in 1883 were 

 found to belong to an acid lava, a pyroxene andesite. 2 



Further, glacial boulders can be satisfactorily identified with rocks 

 in situ by a microscopical examination of their thin sections. Thus 

 Norwegian rocks have been shown to occur as boulders in the 

 Eastern Counties, while Swedish and Finnish 

 rocks are common in the drift of North 

 Germany and Saxony. 



We now come to the discussion of the 

 metamorphism to which all rock-masses are 

 liable. The metamorphism caused by atmo- 

 spheric agencies results in decomposition and 

 disintegration. The constituents are, of 

 course, very differently affected, but rapidity 

 of disintegration demands the decomposition 

 of one of the principal constituents. Such a 

 eonstituentisfelspar^hichdecoinposesxmder 

 posited on the surface the influence of water charged with carbonic 

 (After Dr. Sorby.) add into kaolin ; while the products of the 



decomposition of non-aluminous minerals arc 



carbonates, ferric oxide, and quartz. The minute accessory con- 

 stituents, such as the titanium oxides, are not affected by these 

 agencies, and hence are to be found in all clays and sands. 3 At 

 greater depths from the surface disintegration is replaced by the 

 formation of new, especially hydrous, minerals. Thus serpentine 

 is formed from olivine, and sometimes from suitable varieties of 

 augite or hornblende ; chlorite from biotite ; epidote from suitable 

 minerals, and so on. 



Thermal waters charged with various substances are common in 

 all volcanic districts and play their part in the metamorphosis of 

 rocks. In this way a volcanic rock may become silicified through 

 the percolation of such solutions; and microscopical examination has 



1 E. Wethered, Quart. Journ. Geol. Soc. xlviii. (1892), p. 377. 



2 See J. Murray and A. Renard on ' Volcanic Ashes and Cosmic Dust ' in Nature, 

 1884, vol. xxix. p. 585 ; also J. W. Judd, Krakatoa Report, published by the Royal 

 Society. 



5 W. M. Hutchins, however, is of opinion that rutile is produced as a 

 secondary mineral in certain slates, though he would not dispute its occurrence as 

 stated above (Geol. Mag. 1890, p. 264). A series of papers bearing on the subject 

 which he has published since that date in the same periodical are all worthy of 

 careful study. 



