6\ Ascension. 



PART I. 



crystallisation, and some line, white, granular, crystal- 

 line matter which is probably feldspar. Minute frag- 

 ments of this rock are easily fusible. 



Fourthly. — A compact crystalline rock, banded in 

 straight lines with innumerable layers of white and 

 erav shades of colour, varying in width from the ^Vth 

 to the 2Tr7r tn of an inch ; these layers seem to be com- 

 posed chiefly of feldspar, and they contain numerous 

 perfect crystals of glassy feldspar, which are placed 

 lengthways : they are also thickly studded with micro- 

 scopically minute, amorphous, black specks, which are 

 placed in rows, either standing separately, or more 

 frequently united, two or three or several together, into 

 black lines, thinner than a hair. When a small frag- 

 ment is heated in the blowpipe, the black specks are 

 easily fused into black brilliant beads, which become 

 magnetic, — characters that apply to no common mineral 

 except hornblende or augite. With the black specks 

 there are mingled some others of a red colour, which are 

 magnetic before being heated, and no doubt are oxide 

 of iron. Round two little cavities, in a specimen of 

 this variety, I found the black specks aggregated into 

 minute crystals, appearing like those of augite or horn- 

 blende, but too dull and small to be measured by the 

 goniometer ; in this specimen, also, I could distinguish 

 amidst the crystalline feldspar, grains, which had the 

 aspect of quartz. By trying with a parallel ruler, I 

 found that the thin gray layers and the black hair-like 

 lines were absolutely straight and parallel to each 

 other. It is impossible to trace the gradation from the 

 homogeneous gray rocks to these striped varieties, or 

 indeed the character of the different layers in the same 

 specimen, without feeling convinced that the more or 

 less perfect whiteness of the crystalline feldspathic 

 matter depends on the more or less perfect aggregation 



