396 



/. Eights on the New South Shetlands, 



Clusters of these columns are occasionally seen reposing' on 

 their side in such a manner as to exhibit the surfaces of their 

 base distinctly, which is rough and vesicular. When this is the 

 case they are generally bent, forming quite an arch with the 

 horizon. Where they approach the conglomerate for ten or 

 twelve feet, they lose their columnar structure and assume the 

 appearance of a dark-colored flinty slate, breaking readily into 

 irregular rhombic fragments : this fine variety in descending 

 gradually changes to a greenish color and a much coarser struc- 

 ture, until it passes into a most perfect amygdaloid, the cavities 

 being chiefly filled with quartz, amethyst and chalcedony. 

 Sometimes an interval of about forty or fifty feet occurs between 

 these columns, which space is occupied by the amorphous variety 

 elevated to some considerable height against them ; their edges 

 in this case are not at all changed by the contact. 



The basis rock of these islands, as far as I could discover, 

 is the conglomerate which underlies the basalt. It is composed 

 most generally of two or three layers, about five feet in thick- 

 ness each, resting one on the other and dipping to the southeast 

 at an angle of from twelve to twenty degrees. These layers are 

 divided by regular fissures into large rhombic tables, many of 

 which appear to have recently fallen out, and now lie scattered 

 all over the sloping sides of the hills, so that the strata when 

 seen cropping out from beneath the basalt, present a slightly 

 arched row of angular projections of some considerable magni- 

 tude and extent. 



These strata are chiefly composed of irregular and angular 

 fragments of rock, whose principal ingredient appears to be green 

 earth, arranged into both a granular and slaty structure, and united 

 by an argillaceous cement ; the whole mass when moistened by 

 the breath giving out a strong argillaceous odor. The upper 

 portion of this conglomerate for a few feet, is of a dirty green 

 color, and appears to have been formed by the passage of the 

 amygdaloid into this rock, the greenish fragments predominating, 

 and they are united to each other principally by a zeolite of a 

 beautiful light red or orange color, together with some quartz 

 and chalcedony ; a few crystals of lime cause it to effervesce 

 slightly in some places. These minerals seem in a great measure 

 to replace the earthy cement. In descending a few feet farther, the 

 green fragments gradually decrease in number and become com- 

 paratively rare, the minerals also give place to the cement until 

 the whole mass terminates below in a fine argillaceous substance, 

 with an imperfect slaty structure and a spanish-brown aspect. 



This rock being much softer in its nature than the basalt and 

 more affected by decomposing agents, the number of fragments 

 are consequently greater in proportion, and much more finely 



