SAINT MATTHEW, HALL AND PINNACLE ISLANDS. 137 



similar to those last described, and are very probably of about the same 

 horizon throughout. The pale colored stratum referred to reappears at 

 several places, and always with a low southerly dip away from the sea. 



The south side of Saint Matthew island was clearly seen from the sea 

 and closely examined through the telescope westward to about abreast 

 of Pinnacle island. Its general features and the appearance of its rocks 

 are in every way similar to those of the north side. 



The general structure of Hall island, in which the same rocks are con- 

 tinued, is illustrated by the subjoined diagram, sketched along its east 



coast. 



The principal dip is here to the northward at low angles, and the rocks 

 consist of a series of " porphyrites," with intercalated tuffaceous and 

 agglomeritic beds. The rocks shown at a, d and / in the diagram, con- 

 sist preponderantly of grayish purple, purple and gray porphyrites, 

 nearly massive, but sometimes with a rude, irregular, columnar structure, 

 particularly toward the north end of the island, where the rock is dis- 

 tinctly an augite-porphyrite. They have been considerably altered and 



Figure b.—Diagramatic Section along the east Coast of Hall Island. 



decomposed, frequently largely silicified by subsequent solfataric action, 

 while they are often markedly rusty in irregular bands. C 'and e are 

 pale gray in color, and consist of tuff, volcanic ash or fine agglomerate, 

 which is evidently water-bedded, and in some places rather finely strati- 

 fied. This is particularly the case in respect to the bed c, which is from 

 200 to 300 feet thick, and is underlain by a similar thickness (6) of coarse 

 brownish and blackish agglomerate, holding some fragments from one to 

 three yards in diameter. 



The section evidently represents the results of volcanic action Avhich 

 has been in part or altogether submarine. The rocks are everywhere 

 much fractured and jointed, giving rise under the action of the sea to 

 bold, rugged and picturesque cliffs like those of cape Upright. Much 

 chalcedony and jasper occur on the east side of Hall island near its 

 southern end in the porphyritic rocks, and these minerals here compose 

 a considerable portion of the worn beach pebbles in some places. The 

 chalcedony is generally white or milky, the jasper red or red veined 

 with 3^ellow. 



In 1791 Sauer, of BiUing's Russian Scientific Expedition, landed on 



