MISCELLANY. 
393 
by fire on the surface, under the ordinary weight of the atmosphere. A familiar 
example will best illustrate my meaning. Chalk consists of Lime combined 
with carbonic acid; and as for agricultural and other economical purposes, it 
is desirable to have the Lime in its pure state, the Chalk, or Limestone, is 
exposed to a great heat, in kilns erected in the open air, until all the carbonic 
acid is dissipated, and the Chalk is said to be burnt into quicklime. Nov/ it may 
readily be conceived, that if this operation were conducted under such a degree 
of pressure that the gas could not escape, the formation of quicklime would not 
take place; the Chalk would be fused; the carbonic acid, released from its 
present relation with the calcareous particles, would enter into other combinations, 
and the mass when coolsi would be wholly different from the product of the 
Lime-kilns, formed by the same agency in the open air. Experiments have 
proved that this opinion is correct. Sir James Hall exposed pounded Chalk to 
intense heat, under great pressure, audit was fused, not into Lime, but crystalline 
Marble: and shells inclosed in the Chalk underwent the same transmutation, 
yet preserved their forms. That analogous changes have been effected by a 
similar operation in nature, we have abundant proof; hut in this stage of our 
inquiry it is only necessary to remark, that where ancient streams of lava have 
traversed Chalk, the latter invariably possesses a crystalline structure. We shall 
hereafter find, in accordance with the beautiful and philosophical theory of Dr. 
Hutton, that all the strata have been more or less modified by heat, acting under 
great pressure .and at various depths; and that the present position and direction 
of the materials composing the crust of the globe, have been produced by 
the same agency.* The Kuttonian theoiy, indeed, offers a most -satisfactory 
explanation of a great proportion of geological phenomena, enabling us to solve 
many of the most difficult problems in the science; and it is but an act of justice 
to the memory of an illustrious philosopher, and of his able illustrator, Professor 
Playfair, to state that this theory, corrected and elucidated by the light which 
modern discoveries have shed upon the physical history of our planet, is now 
embraced by the most distinguished geologists. 
[The above, we trust, will be considered a satisfactory reply to the queries 
of our Cheshire correspondent on this subject. Should he, however, require 
further explanation, we shall be happy to hear from him again.— Ed.] 
Synopsis of the Bibliographical History of Geology. —The first theory of 
Geology (by seme called “ Geognosy''’) deserving especial attention is perhaps 
that contained in tile TeUztrius Theoria Sacra , by the Rev. Dr. Burnet. This 
eminent philosopher, however, indulged too much in the pleasures of imagination 
to merit much notice at the present day. The “ elegant romance’’ of the worthy 
* See Playfair’s Illustrations of the Huttonian Theory , Yol. I., p. 33, et seq., Edinb., 1822. 
