162 Professor Blum on Pseudomorphic Minerals. 



regions, the air naturally becomes drier ; in the region of calms, 

 on the contrary, it rains constantly, because the lower warm currents 

 in the act of rising lose some of their heats and therefore allow the 

 moisture they contain to be precipitated. Every place between the 

 tropics has therefore a dry season whilst under the influence of the 

 trade-winds, and a rainy season while the calms prevail, — " a time of 

 sun and a time of cloud," as the Indians near the Orinoco say. In 

 the higher regions of the atmosphere, the ascending air flows back 

 towards the poles. We see it often in the light clouds which are 

 attracted towards the lower trade-winds ; yes, we even reach this 

 upper opposing current when we ascend high mountains, such as the 

 Peak of Teneriffe, on Mawnaroa, on the island of Hawaia. More 

 strongly still is the influence of this returning upper current seen 

 in volcanic eruptions. 



(To be continued in our next Number.) 



Gieseckite and Bergmannite (Spreustein), two Pseudomor- 

 phoses of Transformation from Nepheline. By Professor 

 J. It. Blum, of Heidelberg. Communicated by the Author, 

 from Poggendorff. 



At Jyalikko-Fiord, not far from Julianenhaab in Greenland, 

 there occurs, implanted in drift porphyry, a mineral in the 

 form of hexagonal prisms, generally known by the name of 

 Gieseckite. From the similarity of its form, and for the 

 most part amorphous condition of its mass, it was formerly 

 regarded as a variety of Pinite, with which its chemical 

 composition to a considerable extent coincides. It was 

 afterwards grouped by Tamnau with Elaeolite (Nepheline), 

 with which it has the same crystalline form, and is said to be, 

 in its fresh condition, as regards hardness, specific gravity, 

 and lustre, identical. But most of the crystals are found 

 in an altered condition, and coincide neither with the qua- 

 lities just mentioned, nor with the chemical composition 

 of the Elaeolite. The cause of this is to be sought in the 

 alteration which the Elaeolite has undergone. A short time 

 since there came into my possession a crystal of this sort, 

 which, upon my breaking off a small portion at one end, in 

 order to observe the nature of its interior, was seen to con- 



