1851.] On a Series of Calderite Rocks. 207 



This fact of the little fall of the Barometer until the centre was 

 close upon the ships may be connected with the limited extent of the 

 Cyclone itself. I have elsewhere remarked* that for a distance of 100 

 to 150 miles from the centre, there seems to be no exact rates of fall by 

 which we can estimate the distance of the centre ; and in this the 

 Cyclones appear to assimilate to the smaller tornado-Cyclones, tornados 

 and whirlwinds, which, we have reason to believe, do not affect the 

 Barometer to any remarkable extent on their approach. The vigilant 

 seaman and the philosopher will see in this a strong argument for a 

 careful investigation and registration of the various atmospheric signs 

 upon which I have so much insisted, until we can obtain a complete code 

 of these also, and furnish all observant seamen with a Barometer of 

 Signs. 



On a Series of Calderite Rocks. — By Henry Piddington, Cura- 

 tor Museum Economic Geology, 



My analysis of Calderite (in Vol. XIX. p. 145, of the Journal) was 

 of course performed from the best and most homogeneous (i. e. 

 approaching to perfectly mixed) specimen which could be obtained. 

 It will nevertheless be remarked in that paper, which describes the 

 specimen analysed as the type of the rock, that, with the magnifier the 

 siliceous mixture which constitutes it a rock is distinctly seen in thin 

 splinters when held against the light. I have been able, from the late 

 Mr. "Williams' abundant supply of these so called Iron ores, the whole 

 of which are Calderite, to obtain a very curious and instructive series of 

 specimens, illustrating distinctly the formation of the Calderite by the 

 mixture of the siliceous with the Iron and Manganese mineral, like the 

 large and small grained granites, in the first of which the silex, felspar 

 and mica seem rather to be agglomerated in masses than to form a 

 true compound rock, and in the latter the component parts are only 

 seen by a magnifier. I have therefore selected for our Museum, and 

 for the guidance of geologists and mineralogists a series of fourteen of 

 these specimens. I think it right to give a brief description of each, 

 so as to enable them to recognise the rock in its various appearances. 



* Horn Book. 



