100 Dr. Berger on the physical Structure 



rock is argillo-ferruglnous,* and by itself does not effervesce with 

 acids, as it is easy to prove by making use of pieces of the pure 

 sandstone for that purpose ; but it produces so brisk an effervescence 

 from the intimate mixture of^calcareous particles, that it might be 

 very easily mistaken for limestone. The substances which enter 

 into the composition of this conglomerate are numerous ; and it may 

 first be remarked, that these pieces are of very different sizes 

 and forms, sometimes rolled and rounded, sometimes pointed with 

 sharp angles, from very minute grains to the size of several inches 

 in diameter. There are found in it rhomboidal crystals of calcareous 

 spar, and crystals of felspar, most frequently of an opaque white, and 

 decomposed ; pieces of flint ; grauwacke ; yellowish limestone ; 

 rolled masses of a species of porphyry somewhat resembling the 

 antique, the base of which is of a reddish brown colour, not effer- 

 vescing with acids, and containing numerous, small and well-defined 

 crystals of felspar imbedded in it; pieces of a rock which is itself com- 

 pounded, having the appearance of a porphyry, the base earthy, and 

 including small grains of quartz, crystals of felspar, and pieces of 

 bluish carbonate of lime ; and a whitish tender steatite, in small an- 

 gular fragments. 



Poucham quarry is situated about two miles N. N. W. of Exeter. 

 It is of much less extent than that in the parish of Heavitree. I 

 was not able to determine either the direction or inclination of the 



• May not the presence of iron in this conglomerate be owing to the disintegration 

 of the granite of the low chain of Cornwall, the mica of which coatains so great a pro- 

 portion of it in the state of oxide ? 



The Count de Bournon appears even disposed to believe, that the primitive rocks hare 

 been the only source from which the iron has come, that is spread over the whole sur- 

 face of the earth. Traitede Min. vol. I. p. 62. 



