1830 .] 
and Mineralogical Observations . 
361 
completely detached and imbedded in it ; the chalk in contact with the basalt or to 
the distance of 8 or 10 feet, is also found to be altered, so as to be crystalline like 
marble, and to become phosphorescent when heated. Beds of columnar basalt, 
alternate with beds of amorphous basalt, or are separated by a thin seam of bole 
or red ochre. These amorphous or compact beds of basalt are described, how- 
ever, frequently to contain a structure of small basaltic columns, radiating from a 
centre. It may here be observed, that the prismatic figure of basalt is quite differ- 
ent from the regular forms of crystallization in simple minerals ; the measure of 
the angles is not constant, or subject to certain laws, as in simple minerals : these 
prismatic fissures or divisions appear, therefore, to be merely a mechanical effort, 
probably in cooling, and the basis of the columns also appear to be neaily always 
directed to that surface at which the cooling may be supposed to have commenced. 
Thus, in veins or dykes of basalt, the columns are found to be transversely 
In the section of England, it will be observed, that a bed of chalk marl foims 
the lower portion of the chalk strata: this has also been found in Ireland, and 
appears to be general j it contains a portion of alumina, about 10 per cent, which 
renders it valuable as a manure, from its decomposing easily when exposed to the 
weather ; and it is more particularly sought after, for a light, sandy soil, to which 
the alumina is also advantageous. This may lead to some mention of the different 
soils, since they are formed by the decomposition and disintegration of rocks ; their 
qualities may in some degree be inferred from the nature of the rocks from which 
they have been produced, and from which, in some situations, they still continue to 
receive a supply. Granite, when unconnected with beds of other rocks, appears to 
give a barren sand, or pebble soil ; particularly when the argile of the feldspar is 
washed away. Gneis, mica slate, and day slate, give a light soil, bat when beds of 
lime form a portion of the whole mass, the soil is always productive. Magnesian 
limestone, is, however, an exception ; for wherever magnesian earth prevails, it 
appears to rendejr the soil sterile, and unfavourable to vegetation ; when alumina 
forms the principal ingredient, as in the rocks in which feldspar predominates, the 
soil is a stiff clay. Granite without mica (pegmatite) is found to give by its de- 
composition the best kaolin or porcelain earth. . . . 
Some of the basalts give a very productive soil, others appear entirely to resist 
decomposite n : the lavas much the same. Those in Auvergne, though of a date an- 
terior to all tradition, are still fresh and uncovered by soil ; whilst two of the 
from Etna, which are known to have been formed by the eruptions of 1187 and 1329, 
have already 12 and 8 inehes of vegetable earth respectively. The progress of 
decomposition in the basalt and lavas, appears to depend, in some degree, on the 
nmintitv of iron, or of alkali, which they contain. ■ 
The decay of rocks, however, and their deposit in tlie vallies and in the sea, is so 
slow as to be little more than observable that such an action is continually m pro- 
grels. and that the ultimate effort would be to transport the whole of the land un- 
Jam tkp ipvcl ot the ocean. • . i 
Professor Playfair, in his illustration of the Hutton. an theory, enters into .a cal- 
culation to she..-, that from the quantity of mud floated down by rivers in floods, 
Seamount ini century carried into the sea, may be equal in bulk to Mount 
Blanc In ’the same work will be found reasons given for believing that the 
surface of the earth has at different periods changed its level, and that such 
changes may be constantly in progress, though so gradual as scaicely to 
“inducting geological investigations, as will have been seen, many branches of 
science are requisite. ' Zoology or comparative anatomy, so as to be able to detea-- 
mine the nature of the bones found, and their analogy to existing species, lias been 
mine tue nature , r • so as to deduce very important results, re- 
employed, particuku > nimals w ’ hich have successively lived upon the surface 
spectmg the change. tl re waS luuc h similarity with the present order 
of our globe ; it es \ &hhsh ^ same, but that they assimilate 
of things, but still scarcely p . J a are found in higher or more 
Sly medbedZ * H been particularly directed to . .he bones 
S in the cbalk beds near Parra, and to those of the annuals found m the dtlu- 
vial beds, as of the Elephant and ' *P^P°|“'“ t “ 1 8 e ' determination of the shells. No 
itdividnalcan fully embrace the 'study of all these branrfies, 
£ ^ |ToS ~ 'itrm> w r iuson ’ s Orgaoic 
Remains, and by examining such collections as m< i y 
