20 THK KUII.DING OF AX ISLAND. 



Tlie line which runs at right angles to the direction of the dip, or, in other 

 words, the line which lies horizontally on the sloping rock-surface, is called the 

 line of strike, or sinijily the strike. At outcrops that is to say, at places where 

 the rockv layer crops out, or appears naturally at the surface of the ground, it 

 is sometimes easier to take the strike than the dip. When the strike has been 

 found a very small exposure will give us the direction of the dip, which is at 

 right angles to it ; hut the anioiud of the di]) remains to he observed. 



Usp: ok the Co.mi'ASs.'^' 



To find the direction of the dip we must use a pocket compass, an indis- 

 pensable companion in our exploring expeditions, and since the cutting we 

 examine may cross the rocks in any direction, some caution is necessary in 

 noting the direction of the dip. It is seldom that a cutting or quarry will jire- 

 sent a face of rock which at once shows us the diji, and we have to observe it 

 on projecting corners, where such can be found, or estimate it from what we 

 see on rock faces having different directions. After a little practice the ama- 

 teur will find it fairlv easv to get the strike and dip of the rocks in anv section 

 exposed. 



In. recording the strike and dip on a map a short straight line is used to 

 show the strike, and another springing from its middle point at right angles 

 shows the direction of the dip ; the amount of the' dip may be recorded by the 

 length of the line, which should be long for a low dip and short for a high dip, 

 or both direction and amount mav be entered in letters and figures close to 

 the line. 



But to return to the southwest dip, which wc found to be prevalent all 

 along the eastern edge of the Central Slope, we naturally ask whether the 

 same dip is to be found over the whole of this slope? If not, to what extent 

 does it prevail, and what regulates it ? We have noticed that the tilt is from 

 the east side, where the hills of the Eastern Triangle commence, and it per- 

 haps occurs to us that on the western side of the Central Slope, where the 

 rocks rest against the hills of the IVcstern Oblong, it would not be surprising 

 to find a corresponding tilt from that direction that is to say, from the ivcst, 

 and on examining these rocks this is just what we actually do find. For if we 

 study the layers exposed at different points along the tvestern edge of the 



* A poclcet compass can be purcliased for about half a crovvu or 3 francs, and will be found useful 

 for other purposes besides finding the directions of the dips. 



There is a very remarkable fact in connection with the compass which ought here to be noted, not- 

 withstanding that it has scarcely any practical bearing on observations taken in St. Croix, and that is its 

 variation. It happens that in our island the magnetic needle points very nearly to the true north, but in 

 many other places it deparls considerably from that rule. In northwestern Europe, for example, it points 

 to west of north, the variation being in Denmark about 12 degrees and in England 16 to iS degrees. Here 

 in Santa Cruz the variation at present does not amount to two degrees, say the forty-fifth part of a right 

 angle. It is necessary to say " at present" because it is slowly increasing year by year, the annual addi- 

 tion being at Santa Cruz about 3 minutes, or say. the twentieth part of a degree. In the distant future, 

 therefore, it will be necessary, even for an amateur, to make allowance for variation, though at present it 

 is too small to be, in ordinary observations, of any practical conseciuence. 



