THE BUILDING OF AN ISLAND. IO5 



NOTES. 



On Finding the Anticlinal and Synclinal Axes of the Eastern Triangle. 



On page 49 it was shown that on examining the 

 rocks along the cleft called "Spring Gut," which 

 cuts through the Christiansted group of hills from 

 North to South, a change in the dip was found to 

 take place towards the southern end of the valley 

 where the dip passes from about southwest to east- 

 by-north, and that at Longford, in the plain be- 

 low, the dip was found to be east-northeast. First 

 assuming that we have here, at the change of dip, 

 a point in a synclinal axis, and that such axis has 

 the same direction as the similar axis in the north- 

 western oblong, we run out the line, in both direc- 

 tions, and we shall find that, with a very slight 

 modification, it will, in both directions, come out 

 at places where the dip changes from the one class 

 to the other ; that is to say, from dips looking 

 towards the south quadrant to dips looking towards 

 the northeast quadrant. Towards the northwest, 

 along the line, the change takes place close to Bees- 

 ton Hill, where the main road some years ago 

 showed a section roughly sketched in the diagram 

 (Fig. 32). When this diagram was drawn and the 

 following notes made, the writer had no idea of the 

 true significance of the change of dip at this place, 

 but supposed it be merely a local reversion. It was 

 not till long afterwards that he found that the new 

 dip extended over a considerable .strip of country 

 .southwards and indicated the presence of a syncli- 

 nal axis at the place where the change takes place. 

 The notes read thus ; "In repairing the road at 

 a ' and ' b ' in the plan the bank has been cut in- 

 to by the hoe and a clean surface exposed at about 

 six feet below the surface of the soil at the top of 

 the bank. The cut surface shows the usual thin 

 bandsof indurated clay rocks, some of dark colour, 

 but most of various light brown shades. At ' a ' 

 they dip south, at 'b' north. The decomposition 

 is such that the greater part may be called clay, 

 but some beds, though cracked into small frag- 

 ments, retain a certain degree of hardness in those 



fragments. The whole length has been quite 

 smoothly cut with the hoe. On the opposite side 

 of the road the rock also shows strata dipping 

 northwards." 



Later observation showed that the " north- 

 ward " dip was about north-northeast, or north- 

 by-east, and, as already noted, such dips extend 

 over a wide strip of country .stretching south- 

 wards. 



While on the south side of this change at Bees- 

 ton Hill the dips areall to the north, on the other, 

 namely, towards Christiansted, they are all to 

 southwest. Hence it is clear we have here come 

 upon a synclinal axis, and it is the same as was 

 iddicated by the change of dip in the "Spring 

 Gut" Valley. 



Returning now to the middle of our synclinal 

 line and following it to the southeast we find that 

 it separates the rocks at Fareham Point, which 

 dip to northeast, from those at Petronella, which 

 dip to southeast-by-south. Thus the northern 

 boundary of the strip of country where the 

 strata have a northeasterly dip is apparently set- 

 tled. Proceeding next to find its southern bound- 

 ary, we first follow the south coast from Fareham 

 westwards to find out where the dip changes ; and 

 we find south of the old mill at Fareham some 

 stratified rocks of indurated clay dipping east- 

 northeast at about 50 degrees. Hence we are still 

 in the same strip of northeasterly dipping rocks. 

 Farther west, however, we come, on reaching the 

 point at the east end of Springs Bay, to a set of 

 clearly stratified indurated clay rocks dipping in 

 quite a dilTerent direction, namely, to south-by- 

 east, at about 45 degrees. Hence we appear to 

 have passed once more into a district of southerly 

 dips. This discovery is borne out when we follow 

 the road from Longford westwards and find close 

 to the new bridge at Cornhill some well-defined 

 strata dipping at about 60 degrees to south-south- 



FiG. 32. 



