140 



THE GEOLOGIST. 



these of irregular outline is crossed by the Ehine at Andernach. Its 

 western and more important part is about three miles long and two 

 broad. The Laacher See is well known as a lake occupying an old 

 crater in this part of the country, not far from Andernach. This dis- 

 trict was described and illustrated by Dr. S. Hibbert, in 1853, in his 

 'History of the Extinct Volcanos of the Basin of Neuwied,' etc. 

 The other district, characterized by volcanic rocks and craters dis- 

 persed over an area of about four miles by three, is at a short dis- 

 tance to the south-west, and contains several large lake-craters, such 

 as the Gemunder Maar, the Pulver Maar, the Meerfelder Maar, etc. 

 One of the old lava-streams in this area is met with at Bertrich, on 

 the Ees, a small river running into the Moselle half-way between 

 Treves and Coblentz. Consisting of columnar basalt, and being per- 

 forated by a natural aperture, this mass of volcanic rock presents the 

 aspect of'a basaltic colonnade, and has always attracted the attention 

 of travellers, especially as the joints of the basalt, instead of taking 

 a regular polygonal or angular shape, are more or less spheroidal, 

 " so that a pillar is made up of a pile of balls, usually flattened 

 hence the grotto at Bertrich is called the Kasegrotte, or Cheese-cave. 

 " The basalt is part of a lava-stream, from thirty to forty feet thick, 

 which has proceeded from one of several volcanic craters still extant 

 on the neighbouring hills ;"t and, having run in the valley, it has 

 since been partially destroyed and excavated b}^ long-continued water- 

 action. Mr. J. E. Lee has favoured us with a pencil sketch of this 

 interesting Cheese-grotto, from which Plate YIII. has been engraved ; 

 and, although the grotto is well known to geological students by the 

 woodcut in Sir C. Lyell's ' Manual,' p. 386, yet we think that as 

 the subglobular character of the basalt is very much better shown by 

 our correspondent's sketch than in the little woodcut alluded to, we 

 shall be doing good service by producing it here. 



In connection with the Cheese-grotto, Sir Charles alludes to the 

 occurrence and characters of globular lavas and trap-rocks, adducing 

 particularly the globiform pitchstone of Chiaja di Luna, described and 

 figured by Mr. Poulett Scrope, in his account of the Ponza Isles (Geol. 

 Trans., 2nd ser. vol. ii.). This pitchstone has the globiform structure 

 near its junction with prismatic trachyte; and itself shows a ten- 

 dency to the columnar division ; the columns, however, separating 

 into large globes or ellipsoids, placed one upon another, and, when 

 weathered, ]'eadily desquamating at a touch into numerous concentric 

 coats, having a kind of onion-peel structure. Dilferent degrees of 

 the ])rismatic or columnar condition, passing into the concentric and 

 nodular, iwc observable in many basaltic and trachytic lavas, as well 

 as in o\dcv trap-rocks (diorites, etc.) ; and indeed granite not unfre- 

 quently shov. s a tendency to split and exfoliate in a similar manner. 



The explanation of the columnar and nuclear structure is well 

 given on Mr. P. Scrope's ' Considerations on Yolcanos,' etc., 1825 

 (an admirable work, now out of print, but about to be revived, we 

 liope). In chapter 0, p. 134, etc., the divisionary structure assumed 

 * Lvcll's Mauiuil Geol. p. 3S7. f Ibid. 



