THE BUILDING OF AN ISLAND. 55 



At the estate " Southgate " and the neighbouring islet of " Green Kay," 

 there is a great quantity of a grey rock which, in appearance, resembles granite, 

 being composed of white crystals with black oblong crystals among them. It 

 is, however, different from granite, for while that rock consists of the minerals 

 felspar, quartz and mica, the rock at the places named consists of felspar and 

 the dark mineral hornblende. On Green Kay, both the black .and the white 

 crystals attain in some places an unusually large size. The rock is called 

 gfreenstone or diorite. 



Professor Cleve mentions the occurrence of diorite at Southgate and 

 Green Kay as well as elsewhere among the islands, and has some interesting 

 comments on the rock. He regards it as altered from the sedimentary rocks 

 by heat to such an extent that it has in some parts, but not everywhere, 

 become actually molten. He arrives at this conclusion from having found it 

 in the Virgin Islands, both interstratified with the other rocks without any 

 sign of intrusion, and also filling cracks in the surrounding rocks, in which 

 latter case it has evidently been intruded in a molten state. 



Such a conclusion is probablv true of at least some other intruded rocks. 

 There is nothing to show that they come from any considerable distance lielow, 

 and it may well be that they are sometimes only the stratified rocks of the 

 neighbourhood which have actually been melted and thus made capable of 

 passing among the other rocks, following the direction of least pressure. 



These rocks which we have seen to be thrust in among the stratified rocks 

 of our older formation in masses and dykes, though of volcanic character, are 

 not, strictly speaking, volcanic rocks, that is to say, they have not been thrown 

 out bv volcanoes. So far as the present writer is aware, there is no evidence 

 in anv part of our island of volcanic outbursts through the rocks we now see. 

 There is a strong probability, it is true, that the existing layers have been 

 formed from matter thrown out by volcanoes, either directly into the sea or on 

 to a neighbouring land from which they were washed into the sea, but there 

 appears to be no evidence anywhere of a crater or of the piling up of layers of 

 volcanic ash and cinders. If there were volcanoes at any time on the site of 

 what is now Santa Cruz, they have long ago been swept away in some of the 

 many changes that have passed over the solid structure of this beautiful little 

 land of ours, and no trace whatever of them remains. 



At the same time the rocks in question, though not likely to have been 

 actually volcanic, consist of the same substances as make up the volcanic 

 rocks, and, as already noted, they have been in a molten condition. 



Rocks of this class are often spoken of under the general name of " trap," 

 an old-fashioned name but convenient for an amateur, since its use avoids the 

 necessity of any closer naming of the kind of rock, a task which must be left 

 to experts. The name "trap" is derived from the Swedish " trappa," a stair, 

 and was applied to this class of rocks because they are sometimes found to be 

 arranged in successive steps, formed from outflows whose spread became less 

 and less as they were successively pushed up amongst the strata, lifting the 



