42 THE DUILDINC OK A.N ISLAND. 



faces being flat. The planes of divisions are technically called "joints." The 

 tendency to split in this way is very common, and generally becomes con- 

 spicuous when the rock is acted on by the weather ; in some cases, however, 

 the rock is solid and very hard, and onlv comes awav in large blocks. Some- 

 times the jointing is so regular in parallel planes in a given direction that we 

 have to be on our guard not to mistake it for stratification. 



Perhaps the most remarkable change of all, a change allied, may be, in its 

 nature to that mentioned above, is that which has taken place in the slate 

 rocks. Essentially these rocks resemble in composition the other rocks of the 

 older formation, but the texture is very fine, and thev have underafone a 

 process which has originated a structure called cleavage. Cleavage is a 

 peculiar property that distinguishes slates from all other clay rocks. It is the 

 tendency to split along parallel surfaces into slabs of varying thickness according 

 to the (]ualitv of the rock. These cleavage surfaces or planes do not follow 

 the planes of stratification but are often placed at a considerable angle to them. 

 They are, in fact, quite independent of the way in which the material has been 

 put down, and have arisen from the enormous pressure to which that material 

 has been subjected later. In the well-examined slate districts of European 

 countries and North America, it has been shown that the direction of the 

 cleavage depends on the direction of the neighbouring axis of elevation; it 

 follows in a general way the direction of that axis, from which it may be in- 

 ferred that the pressure which has forced up the wave of the earth's crust, 

 known as an anticlinal, has also pressed on the clay rocks with such intensity 

 that they have rearranged their particles in the form of adjacent plates. The 

 fact that pressure has caused this rearrangement cannot, so far as the present 

 writer knows, be shown in the St. Croix slates, but in other lands, where fossil 

 shells arc sometimes found embedded in the slate, it is seen that the shells are 

 lengthened in tiie direction of the cleavage and shortened in the cross direction ; 

 they have, in other words, been squeezed out of shape and always in the way 

 described. What is thus proved for many slate formations mav be assumed 

 as true of ours. Professor Cleve, writing of the clay-slate of St. Croix, savs 

 that "it is perfectly cleavable," that is to say, he found that he could split il 

 along the cleavage planes in the same way as the European or American slates 

 are split ; but this must not be taken to mean that they have any commercial 

 value ; the beds are neither thick enough nor fine enough for that. The 

 cleavage is also most generally irregular, as far as the present writer has been 

 able to ascertain, and the slates are frequently in thin beds, and these are often 

 interstratified with other rocks, which show the cleavage in a less distinct wav. 



It may also be noted that the planes of cleavage in the slates are some- 

 times shown by fine white lines along the surfaces of splits in the rock; and 

 the edges of the thin slabs occasionally appear as tiny parallel waves. Thus it 

 will be seen that the cleavage is a very interesting feature of the slate rocks, 

 and leaves room for extensive study, equally with other changes of which 

 evidence has been observed, such as the crystallization, the formation of con- 

 cretions and the jointing. 



