148 ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 



A consideration of the whole subject leads us to the con- 

 clusion that the effects of joints and faults upon scenery can 

 hardly be over-estimated, and the more the scenery is studied in 

 detail, the more evidently will this be recognized. It is quite 

 natural that this should be so, as a little consideration will show. 



A joint is, from the nature of the case a line of weakness, 

 and where these most abound there will be the greatest sus- 

 ceptibility to denudation. Mechanical denudation will be 

 facilitated because the rock is so much more readily abraded and 

 reduced to pieces small enough to be removed, and chemical 

 destruction because the amount of surface is proportionately so 

 oreatly increased. In this way depressions of the surface and 

 indentation of the coast line are to a great extent produced. 



Sometimes however, and very frequently where joints are 

 developed into fissures, the rocks become greatly hardened 

 (silicified) by the circulating mineral solutions. In such cases 

 they are often so well able to resist denudation as to stand up 

 above the general surface of the country. 



When the joints dev elope into fissures, and finally into 

 faults, the surface irregularities are heightened by the juxta- 

 position of dissimilar rocks which is thus occasioned. 



A depression -once formed tends constantly to enlargement, 

 because the rain-wash becomes concentrated in it, and finally, 

 when the valley has been cut down nearly to sea level, alluvial 

 deposits are laid down in it, which hide the faults under 

 accumulations of detrital matter. 



The foregoing remarks apply exclusively to the scenic effects 

 of faults and joints ; in a future chapter reference will be made 

 to the super-added effects due to the presence of ore deposits. 



Faults and Geological Maps. In all accurate geological maps 

 which are drawn on a moderately large scale, the positions of 

 the various faults are clearly indicated by abrupt interruptions 

 of the lines which bound the various colours, and often by the 

 simultaneous termination of two or more colours. A moment's 

 reflection will show that this must necessarily be the case ; it is 

 in fact so general, that assuming the accuracy of the surface 

 colouring, when such abrupt change or terminations occur, we 



