The Geology of Ascension and St. Helena Islands. 151 



But another fact stimulated the wish to explore St. Helena. 

 Reinisch, in the paper above noted, states that a graywacke, con- 

 sisting of many pieces of quartzitic schist, quartzite, sandstone, 

 granite, gneiss, and other " continental rock types ", had been 

 found in the " Laing-Berge " of St. Helena. This announcement is so 

 weighty in the theory of Gondwanaland that further data of the kind 

 seemed desirable. So far as that object is concerned, the writer has 

 had negative results. In spite of constant search, neither the gray- 

 wacke nor any other quartz-bearing or " continental " rock was 

 found. " Laing-Berge " is a locality unknown to St. Helenians. 

 Is it possible that Reinisch's specimen was brought from. near 

 Laingsburg, South Africa, by the same Boer prisoner, interned at 

 St. Helena, who collected the specimens of St. Helena lavas described 

 by Reinisch '( 



St. Helena is largely composed of basaltic rocks, olivine-bearing 

 and olivine-free. They will probably be found to duplicate some of 

 the Ascension types. These rocks form superposed, generally thin, 

 flows, which with thin interbeds of tuff and rarer agglomerates 

 constitute the greater part of the island ; trappean, vesicular, 

 and porphyritic dykes cutting the flows are exposed in hundreds ; 

 several large areas of remarkable complexes, made up of thick masses 

 of tufE which are cut by multitudes of simple dikes, many multiple 

 dikes, and many dike networks ; a number of volcanic necks, 

 and monolithic crater-fiJlings ; and at least one sill. 



The tufis aiid associated agglomerates of the complexes are the 

 oldest rocks. In some resjDects the most notable complex is that 

 covering about 1'5 square miles at the north-east corner of the island, 

 stretching from the middle of the shore of Flagstaff Bay, between the 

 line of heights from Flagstaff Hill, through Woody Point to the Turk's 

 Cap on the one side, and The Barn on the other. This may be called 

 the Knotty Ridge Complex, from the name of a prominent feature in 

 the broad belt. Darwin (in his Volcanic Islands) and Mellis (in 

 St. Helena, London, 1875) give good descriptions of it, but even 

 the previous reading of their books did not prevent astonishment at 

 the actual profusion of dikes. These trend in all azimuths. The 

 more conspicuous dikes are grouped in two systems. The one com- 

 prises dikes, generally vertical or nearly so, whose outcrops run in 

 a north-east — south-west sense, the strike varying from north 

 20°-30° east (common readings) to north 80° east. The average 

 trend of this system, specially well seen from the ridges south-west 

 of The Barn, agrees fairly well with the more regular north 40°-45° 

 east trend of the main dike system in a similar basal complex of the 

 Sandy Bay district. The second system of the Knotty Ridge dikes 

 varies in strike from north 10°-15° west (common readings) to about 

 north 10° east. Many dip 70° or even at 60° to the westward, 

 indicating unusual conditions of tension in the older rocks. The 

 dikes range in width from an inch or less to a score or more of feet 

 in width ; the average is perhaps four feet. 



