81 



On the FossiU of the Eastern Portion 



[July 



have been produced only by their being formed simultaneously. I have 

 seen masses of calcedony, passing into a black mineral not to be dis- 

 tinguished from the surrounding basalt. They also occur in the com- 

 pact nodules where no fluid could have had access, and even in the 

 cavities of the Seetabuldee basalt, lined by an impervious glassy coat. 

 In this rock, likewise, calc spar occurs, penetrated by needle-like cry- 

 stals of the sam.e substance, invested with a crust of the basalt and con- 

 nected with each side of the cavity, and resembling chiastolite in struc- 

 ture. It is indeed impossible to conceive how many of the appearances 

 presented by the agates, cornelians, or drusy cavities in calcedony, 

 partially or entirely filled by quartz crystals, or a central mass of calc 

 spar, could be formed in the way supposed, as the first coating of silex 

 would effectually close out the further access of the aqueous solution of 

 that substance ; nor could layers so formed, separate into distinct cavi- 

 ties, having both sides covered with quartz crystals, as they are some- 

 times seen to do. It is, however, only by a careful study of the rocks 

 themselves, that this can be fully understood. I have selected a few 

 specimens to show that the majority, if not all the minerals of the Indian 

 trap rocks, are not formed by infiltration. One of these, perhaps, deserves 

 particular notice ; it is a mass of calcedony, 8 or 10 inches in length and 

 6 or 8 in diameter, of a conical shape, and was found imbedded with its 

 apex downwards between the globular basalt, and impressed with ihe 

 iiTegularities of its surface. In another of these specimens the upper 

 part is perfectly flat and smooth, without any impressions of the basalt, 

 and is composed of a thick covering of cacholong, parallel stripes of 

 which appear to indicate the slow cooling of the surface. The centre of 

 the mass is composed of quartz crystals, radiating to the centre, which 

 is occupied by calcareous spar impressing or impressed by the quartz. 

 The mode of its occurrence increases the conviction in my mind, that the 

 only correct theory of the formation of such minerals in trap rocks, is the 

 play of the molecular attractions existing between similar particles of 

 matter. That so eminent an inquirer as Dr. Turner should have asserted 

 that all calcedonies, rock crystals, &c., even when occurring in volcanic 

 rocks, are the result of aqueous infiltration*, I can only account for, by 

 the diflaculty of explaining how carbonic acid is retained at high tempe- 

 ratures, in any other way than by supposing a great pressure to have 

 existed at the time the rock was in an ignited state ; but of the existence 

 of which, proof is often entirely wanting where crystals of carbonate of 



• Lecture on the Chemistry of Geology, by Dr. E. Turner, Ediu. New Phil. Journal, 

 Oct., 1833. 



