52 GEOLOGICAL MEMOIRS. 
both of them upwards must have found in the interior of the mountain 
an ancient mass of leucite lava in a soft condition, have torn it to pieces, 
and separated the more infusible leucite crystals from the softer 
lava paste, and ejected both separately. And in reality there occur 
on the Punta dei Minatori on Monte Somma, as well as under the 
town of Pompeii, old easily comminuted leucite porphyries, containing 
crystals exactly similar in form and size; whilst the author does not 
remember to have seen such im the newer beds. Hence the opinion 
is easily refuted that the leucite mass was ejected in a fluid state, and 
first formed into crystals in passing through the atmosphere. Equall 
erroneous is the assertion that Vesuvius, on the 22nd of April 1845, 
threw out pyroxene crystals affected by acids : pyroxene in this state 
could only come from the crater itself. 
[J. N.] 
Present and Former Extent of the Island of Heligoland. By M. 
WIEBEL. 
[From the Proceedings of the Association of German Naturalists at Kiel in 1846. 
L. and B.’s Jahrbuch, 1848, p. 82.] 
Ir appears (1.) that the well-known map of Heligoland by Meyer, ~ 
according to which the island once contained nine parishes, is en- 
tirely a work of the imagination; (2.) that on comparing the map 
made in the year 1793 by the Danish engimeer Wessel, of which 
however only a three-inch reduction remains, with the author’s own 
measurements, “the coefficient of destruction in a century for the 
whole circumference of the rock washed by the sea does not on the 
average amount to more than three feet ;’ (3.) that m the time of 
Adam of Bremen (an extended description by whom is still in 
existence), and of Charlemagne, the island was only a little larger 
than at present. 
[J. NJ 
Travels in Northern Persia. By M. Woskosotnixow. 
[Erman’s Archiv. fiir Russland, vol. v. p. 671. Jahrbuch, 1848, p. 96.] 
Tue following are the chief geological results regarding the northern 
half of Persia examined by the author. 
1. The system of limestone beds with marls, and of subordinate 
green sandy marls, appears to be the oldest of the formations occur- 
ring here, and indeed to be older than the mountain limestone. 
From the want of petrefactions, however, their character cannot be 
more precisely determined. 
2. The beds of the coal formation and the metamorphic strata 
dip mostly to the W.S.W. In the mountains on the coast,-however, 
all the beds of the other formations are inclined towards the sea. 
3. The cretaceous and nummulite strata only appear on the 
