494 PROF. J. W. SPENCER ON TELE GEOLOGICAL AND [NoV. I9OT, 



IV. The Seaeorth Limestones. 



At a few points in the valleys of the mountain-zone, as at 

 Seaforth and the adjacent estate of York (which I visited along with 

 Mr. Porrest), there is a very compact dark-grey limestone, in 

 appearance resembling old formations, even of Palgeozoic age. The 

 beds dip at considerable angles south-westward. Only fragments 

 of these deposits remain, in protected places, on account of sub- 

 sequent denudation. This appears to be the oldest formation 

 succeeding the igneous rocks, although this limestone has not been 

 found eastward of the mountain-zone, beneath the stratified 

 rocks which overlie the old trappean deposits. Nothing can be 

 said as to its age, for no fossils were found in it, but it resembles 

 limestones of other islands, and on this account, and because of its 

 very different character from the Tertiary formations, it is possible 

 that it may be as old as the remains of the Cretaceous Period, such 

 as are seen in the island of St. Croix. 



V. The Tuffs and included Marine and Presewater Cherts. 



Extending diagonally across the island, in a belt from 2^ to 

 4k miles wide, and occupying the central plains and the north- 

 western flank of the mountain-zone (up to an altitude of 695 feet 

 in the case of the isolated hill called Monk's Hill), there is a very 

 thick formation of stratified tuffs. These beds dip from 12° to 20° 

 north-eastward. 



The tuffs are made up of more or less decomposed angular 

 and subangular fragments of trappean rocks, and contain minute 

 crystals of felspar and magnetite. While the tuff's of fine texture 

 predominate, near the mountains they become a conglomerate. In 

 some cases the particles are water worn. The colour is greenish 

 to brown, and even whitish where kaolinization is more complete. 

 These tuffs appear to have been derived from the denudation of the 

 older volcanic formation, which extended to the nearest islands 

 (30 miles distant). Drew's Hill,^ situated in the middle of the 

 zone, seems to have been produced by a renewal of volcanic 

 activity, long after the commencement of the epoch of the tuffs, 

 and may have contributed in a measure to the mass of that 

 formation. 



Lying conformably within the tufaceous group are lenticular 

 masses of cherty limestone, containing rounded or fragmentary 

 remains of shells and corals. These rocks give rise to isolated 

 prominences. Higher come vstrata of sands and grits with water- 

 worn volcanic fragments ; and still higher are irregular thin layers 



^ The core of this hill is a compact o-ystalline eruptive, rising vertically 

 through the tuffs to the summit, and is described by M. Purves as a trachy- 

 d o 1 e r i te , containing large crystals of tri clinic felspar, with a pyroxenic mineral 

 and grains of magnetite. It is the presence of the pyroxenic mineral which 

 distinguishes this rock from the older igneous formation. 



