GEOLOGY. 



GEOLOGY. 



countries, where the soil is deep and rich, and 

 consequently requires less moisture, the clouds 

 float undisturbed over the plains, and the coun- 

 try frequently enjoys that long and uninter- 

 rupted series of dry weather which is so con- 

 genial to it. As a general rule it may be stated, 

 that to obtain the greatest fertility the propor- 

 tion of siliceous sand in a soil ought to increase 

 in proportion to the quantity of rain that falls, 

 or rather perhaps to the frequency of its re- 

 currence ; for one of the effects of silex is to 

 diminish in the soil its power of absorbing 

 moisture; we accordingly find that in the rainy 

 climate of Turin the most prolific soil has from 

 77 to 80 per cent, of siliceous earth, and from 

 9 to 14 of calcareous, whereas, in the neigh- 

 bourhood of Paris, where there is much less 

 rain, the silex bears only the proportion of 

 from 26 to 50 per cent, in the most fertile parts ; 

 and I have found some of the most productive 

 corn lands in the parish of St. Burian to con- 

 tain as much as 70 per cent, of that earth." 

 See EARTHS, THEIR USE TO VEGETATIOX. 



It will, perhaps, considerably assist the cul- 

 tivator in his examination of the different 

 geological formations to which he may have 

 to direct his attention, if we examine the che- 

 mical composition of a few of the stones and 

 other substances of which rocks are chiefly 

 formed. 



Common clay is merely a mixture of alumina 

 with silica, in endless proportions. The alu- 

 mina is in the form of a very impalpable 

 powder ; but the silica, says Dr. Thomson, is 

 almost always in small grains, large enough 

 to be distinguished by the eye. Clay, therefore, 

 exhibits the characters of alumina, and not of 

 silica, even when this last ingredient predomi- 

 nates. Besides alumina and silica, clay often 

 contains carbonate of lime (chalk), carbonate 

 of magnesia, carbonate of baryta, oxide of iron, 

 &c. (Chem. vol. iii. p. 341.) 



Loam may be regarded as a very impure 

 potter's clay united with iron, ochre, and mica. 



Common Mica. This stone is composed, ac- 

 cording to the analysis of M. Klaproth, of 



Silica - 



Alumina - 



Oxide of iron - 



Oxide of manganese 



Potash 



Loss - - - 



- 47- 

 -SO- 



- 15-5 



- 175 



- 14 5 



- 125 



100- 



Common feldspar, according to M. Vauque- 

 lin, is composed of 



Part.. 



Silica 6283 



Alumina 17-02 



Lime -3' 



Oxide of iron ...... i- 



Potash ....... 13- 



Loss ----.--_ 3-15 



100- 



The decomposing feldspar of Cornwall is 

 composed, according to Mr. Wedgwood, of 



Parts. 



Alumina - - - . . . .60 

 Silex -----.-.20 

 Moisture and loss - - - . . .20 



100 



Brande found in a specimen of pale flesh- 

 coloured feldspar from the Alps 



100- 



Common hornblende was found by M. Klap- 

 roth to contain 



Parts. 



Silica 42- 



Alumina ------- 12- 



Lime 11' 



Maeneiia 225 



Oxide of iron - 



Oxide of manganese 



Water 



Loss - - - 



- 30- 



- 025 



- 075 



- 1-75 



100- 



Common serpentine contains, according to 

 M. Vauquelin 



Silica - - - 



Magnesia - 



Alumina 



Oxide of chromium 



Oxide of iron 



Oxide of manganese 



Parts. 



- 44- 



- 44- 



- 2- 



- 2- 



- 73 



- 1-5 



100-8 



Another specimen, examined by Dr. John, 

 was found to contain 



Par*. 



Silica 31-50 



Magnesia 47-25 



Alumina ------- 3- 



Lime 0-50 



Iron 5-50 



Oxide of manganese - .... 1-50 

 Water 10-50 



Chalk contains 



Lfme --- 

 Carbonic acid 

 Water - 



99-75 



- 56-5 



- 43- 



- '5 



100- 



Common compact limestone contains 



Lime ... - 

 Carbonic acid - 

 Water - - - 

 Silica - 

 Alumina - 

 Oxide of iron - 



Part*. 



- 53- 



- 42-5 



- 1-63 



- 1 12 



- 075 

 100- 



Magnesian limestone, from Sunderland, con- 

 tains, according to Dr. Thomson (System of 

 Chem. vol. iii. p. 396) 



Parts. 



Carbonate of lime - - ... 56 8 

 Carbonate of magnesia .... 40 84 

 Oxide of iron ----__ 36 

 Clay, water, &c. - .... 2- 



100- 



Quartz, according to the analysis of M. 

 Bucholz, is composed of 



Silica 97-75 



Alumina -.----- 0*50 



Water 1- 



Loss 0-75 



100- 



2 T 2 



