124 Mr. Henderson's Remarki 



and though possessing a tendency to the same form as the 

 outer coat of quartz, not disposed with regularity ; while on 

 the contrary others consist of a mere external coating of 

 smoky quartz, the interior being wholly gray, which in all cases 

 affords brisk effervescence on the application of acid. A por- 

 tion of the limestone taken from the immediate neighbour- 

 hood of a crystal was wholly dissolved, with very brisk effer- 

 vescence. 



It seems reasonable to conclude that such part of the gray 

 substance as does not yield to the action of the acid, is sili- 

 ceous or quartzose ; and that the prime difference between it 

 and the smoky quartz surrounding it, consists in the different 

 circumstances of crystalline aggregation under which they were 

 deposited. 



These crystals are at least curious, if not instructive, and may 

 hereafter, in connection with the somewhat analogous case of 

 the Fontainbleau sandstone, as it is termed, serve to assist in the 

 illustration of some points relative to the laws of affinity as 

 operating in the formation of crystals, when a sufficient num- 

 ber of facts shall have been collected. Meantime, it may 

 perhaps be permitted to observe, that the fact of their including 

 a proportion of limestone would lead to the notion of their 

 formation having been contemporaneous with the deposition 

 of the limestone inclosing them. And it seems not less certain 

 that the menstruum from which the whole was deposited, dif- 

 fered at different and alternating periods ; now permitting the 

 free exercise of that law by which the particles of silex coa- 

 lesced and were deposited with crystalline regularity, and then 

 affording interruptions, by which minute portions of limestone 

 were involved mechanically among the siliceous matter. 



XXIV. Some Remarks on Capt. Sabine's Pendulum Observa- 

 tions. By Thomas Henderson, Esq.* 



IN the Quarterly Journal of Science, Literature and Art, 

 New Series, No. II. p. 382, Capt. Sabine has taken notice 

 of an error which he had committed in estimating the value 

 of the divisions of the level of the repeating circle employed 

 in his pendulum experiments, each division having been as- 

 sumed to denote single seconds in place of ten, the correct 

 quantity. The circle having been used for determining the 

 rate of the chronometer at New York, he has computed anew 

 the observations made there, and has found that the results of 

 the pendulum experiments at that station are affected to a very 



* Communicated by the Author. 



trifling 



