346 Mineralogical Observations on Cornwall. [Nov. 



sometimes probably alternate with, transition rocks, are also 

 chemical deposites. 



5. That sandstone in many instances appears to be a chemical 

 deposite. 



6. That many of the fragments and fragmented appearances 

 which occur in porphyry, limestone, and trap rocks, are of a 

 chemical nature, and of cotemporaneous formation with the 

 rocks in which they are contained. 



I . That true primitive veins, those confined to primitive rocks, 

 whatever may be their nature or magnitude, are often of cotem- 

 poraneous formation witli the rocks they traverse. 



8. That many of the veins in transition and floetz countries, 

 even those extending for many hundred yards, and of great 

 width, are of cotemporaneous formation with the rocks in which 

 they are contained. 



9. That strata of crystallized rocks may appear to run beneath 

 an older rock when they really rest upon it, and have been 

 formed after it. 



tO. That the various wavings in the strata of gneiss, mica 

 slate, clay slate, greywacke, transition slate, and sandstone, are 

 the effects of crystallization. 



II. That the general, physical, and geographical distribution 

 of petrifactions in the crust of the earth does not correspond 

 with that of the present existing races of animals and plants. 



Article V. 



Some Mineralogical Observations on Cornwall. By Thomas 

 Thomson, M.D. F.R.S. 



{Continued frem p. 253.) 



In the rapid sketch of the south of Cornwall, which was 

 printed in the last Number of the Annals of Philosophy, I have 

 described nearly every part of the county that appears demon- 

 strably to belong to the transition class of rocks. 1 suspect, 

 indeed, that this class of rocks extends considerably farther 

 north than I have been able to trace it : nor would it he sur- 

 prising if the whole of the county should be hereafter proved to 

 be transition ; but as 1 observed no facts to support this opinion, 

 we cannot at present adopt it. 



The road from Merazion to Penzance winds along the sea- 

 shore, and no rocks are to be seen till you reach the north end 

 of the latter town. Here a bed of blue stone is perceived 



