THE BUILDING OF AN ISLAND. 53 



CHAPTER VI. 



The Igneous Rocks. 



Our examination of the stratified rocks of the Clay Formation has shown 

 us that a large proportion of them are crystalline, and that the change has been 

 in many instances carried so far that the marks of stratification are not brought 

 out by weathering, and in these cases it is only the gradual passage of the rock 

 from the stratified form to the unstratified form which convinces us that the 

 latter is in reality a sedimentary rock like the former, only that it has under- 

 gone a higher degree of alteration. 



Occasionally, however, we meet with some form of crystalline rock which 

 has plainly been thrust through the stratified rocks, and it is in this case evident 

 that the former has been in a molten condition when it was thus forced 

 through the strata. 



The igneous rocks thus forced through the strata are frequentlv met with 

 in the form of walls of greater or less width, technically known as "dykes." 

 Such dykes are to be seen here and there in our older formation. The most 

 accessible example from Christiansted is a dyke in the quarry immediately east 

 of the town on the south side of the high-road. The stratification of the rocks 

 in this quarry is very plainly marked. They dip at about 45 degrees to the south, 

 and the dyke cuts them through in a wall about 6 to 8 feet thick. Its length 

 lies northwest and southeast, and it slopes at about 60 degrees to southwest. 

 The rock in this dyke is slaty blue or grey in colour, is speckled with distinct 

 crystals and is very hard. Another good example may be seen at Fort Au- 

 gusta, at the entrance to Christiansted harbour. The stratified rocks at the 

 Point are in dark slate-coloured and brown layers alternately, all much con- 

 torted, but mainly dipping at an angle of about 30 degrees to south. Through 

 these rocks a broad dyke passes across the Point north and south showing 

 itself in the clifl^s on both sides. In the yard of the Fort, which at the present 

 writing is bare of gravel, the rock of the dyke may be very plainly seen in con- 

 tact with the beds of the stratified rock, which it has bent backwards, so that 

 they form as they cross the yard a succession of arches facing on the one side 

 to southeast the other to southwest, and in the middle towards the south. On 

 the north side of the Point may be seen embedded in the igneous rock a piece 

 of stratified rock torn off doubtless by the flow of the latter as it broke through 

 the strata. 



There is another good example of a dyke in a "gravel pit," or quarry, on 

 the northeast side of the hill on which " Upper Love " village stands. The 

 rocks here are in regular layers, dipping at a high angle (about 60 degrees) to 

 the east. The quarry looks east, and the dyke passes along its face in a waved 

 line from south to north. The dyke is about eighteen inches wide, is much 

 decomposed on its exposed edge and appears to slope downwards to the west. 

 It has been broken through by small faults (about six inches) in two places, 

 faults which obviously must have been caused by movements later than the 



