30 



THE BUILDING OF AN ISLAND. 



less very interesting and will not escape an observer's attention. It is the 

 occurrence of breaks across the strata, breaks which the geologists call faults. 

 The accompanying sketch, taken from the cliffs at Cane Garden, will show 

 clearly what is meant bv a fault. It will be seen that the whole mass of the 

 strata visible in the cliff has been broken across, and that in each of the three 

 faults shewn, the beds have slidden down a little on one side of the break. Here 

 the slide is of verv small amount, but faults are known to geologists where 

 the difference of level in the beds on opjiosite sides is verv great, and among 

 mountains sometimes amounts even to several thousand feet. In our marl 

 formation there does not seem to have been much cracking and sliding of the 

 strata in this way; at all events very few faults show themselves. 



The name "fault" is perhaps an unfortunate one, but it is easy to see 

 how it has arisen, for when the miners in following up a bed of coal or a 

 mineral vein arc suddenlv stopped by a wall of other beds, not containing the 

 mineral they are working, they naturally regard the arrangement which gives 

 them trouble as a fault. 



Fic. \%. 



Faults in Limestone Rocks ai "Cane Cjauden." 



It will be noticed in the sketch, that it is the masses on the upper sides of 

 the sloping lines of fault which have slidden down. This is the case with the 

 great majority of faults, and arises from the fact that the rocky mass on the 

 upper side of the slope has a smaller base, hence less support, and conse- 

 quently greater tendency to settle down when disturbances of the earth's 

 crust in their locality take place. It is not difficult, however, to conceive of a 

 fault being produced during a time of great side pressure while the rocks were 

 being lifted, and in that case the lighter mass, the mass with the smaller 

 base, would be pushed upwards, and the common ruU' would be reversed. 

 Such faults are called "up-thrust" faults. To that class belong many of the 

 great faults of mountain regions, and examples on a small scale may also be 

 found in the older formation in our own island. 



Thickness of the Limestone Formation. 



An enthusiastic observer will no doubt find many other points of interest 

 in the study of the limestone and marl formation as he proceeds ; here we must 

 be content with only one more enquiry, namely, what is the greatest thickness 

 of the formation as it now exists? This (juestion mav be answered approxi- 



