Vol. 60.] 



IGNEOUS ROCKS AT SPRING COVE. 



159 



olivine-basalt, containing very occasional lumps of limestone, 

 from a few inches to a foot or more across. For the first 100 yards 

 its upper junction with the limestone cannot be seen, because of 

 the accumulated boulders at the foot of the cliff, while the lower 

 junction is covered with water, even at lowest spring-tide. Then, 

 a little more than halfway from the low-water end, and along to 

 the cliff, the basalt changes in character somewhat suddenly. It 

 now contains big lumps of burnt limestone, and the whole mass 

 becomes broken up into a very coarse tuff or agglomerate, con- 

 taining great lenticular masses of highly-slaggy basalt, 5 to 6 feet 

 long, together with lumps and bands of limestone, often considerably 

 fractured, and up to 10 or 12 feet in length. About 20 or 30 

 yards farther on, and as far as the end of the exposure in the cliff, 

 the rock is more uniform in character, being a ' pillowy ' basalt, 

 though considerably brecciated and very amygdaloidal, with com- 

 paratively little tuff. But it still contains masses of limestone, 

 even larger than those in the middle of the exposure. 



Fig. 1. — Lenticles of Java and tuff making 



main 



basalt: 



The whole mass appears to consist of great lenticles of basalt, or 

 tuff, or both confusedly mixed, together with the included lime- 

 stone. The median planes of these lenticles run obliquely to the 

 limestone-beds above and below, so that the lenticles dip at a 

 steeper angle than the sheet as a whole (fig. 1). It would thus 

 appear that the mass is capable of being roughly divided into three 

 portions. Commencing at the cliff-end to the north (in which 

 direction the vent was probably situated), the rock for the first 

 30 yards is a ' pillowy * basalt, with tuff and limestone often 

 occupying irregular spaces between the spheroids of amygdaloidal 

 basalt ; then, for about 20 yards, the rock is mainly a coarse 



