516 Notices of Memoirs — Dr. T.Anderson — Subsidences in Iceland. 



platform some metres square could be reached, which is about half 

 way down the crater. Later the path was further widened by me 

 and made more commodious, and now gives easy access to the plat- 

 form, from which one can look right into the vent of the volcano 

 and watch with ease the boiling up of the lava and the ejection of 

 the great blobs and cakes that are rapidly filling up the crater. 

 Unfortunately, owing to the well-like shape of the crater, the 

 shadows due to the vapour column spreading out overhead, and 

 the dark colour of the rocks, instantaneous photography could not 

 be utilised to record this interesting and everchanging scene. 



As is usual at some period after an eruption, feathery gypsum is 

 a common product in the cavities of the old scoria, and is associated 

 at the fumaroles with a little sulphur (an exceedingly rare mineral 

 at Vesuvius), with abundance of molysite and kermesite. 



In the Campi Phlegr^i little of novelty has come to light. A 

 tunnel and a deep shaft which is being constructed in Naples to 

 complete the drainage works have brought several interesting 

 sections to light, but not of sufficient completeness to be yet 

 worth recording:. 



VI. — On Certain Volcanic Subsidences in the North of Iceland. 

 By Tempest Anderson, M.D., B.Sc, F.G.S. 



PERHAPS the most striking features in Icelandic scenery are the 

 gids (pronounced "geow"), or fissures and chasms, which are 

 so frequently met with in all the districts in which recent volcanic 

 activity manifests itself. They are usually, and in most cases 

 rightly, ascribed to the lower stratum of a molten lava stream, 

 having obtained an outlet after the surface has consolidated into a 

 crust of greater or less thickness. 



Gias of this class are, so far as the author has been able to 

 observe, confined within the limits of a single lava stream, and do 

 not aflfect previously formed rocks. Usually there is a large gia 

 roughly parallel with each side of the original lava stream, and the 

 space between these has subsided considerably. Any gias in this 

 subsided portion are much smaller, and obviously of secondary 

 importance. Examples of this are to be found in the well-known 

 Almanagia, at Thingvalla, which has a throw of about 100 feet, 

 while the sides of the smaller gias which enclose the Logberg in the 

 subsided portion are practically on the same level. 



There are also several such subsidences near Lon and Asbergi, in 

 the north of Iceland. The main subsidence at Asbergi is a little 

 more complicated, though evidently due to the same causes. Here 

 a large roughly triangular area has subsided, the throw at the apex 

 being probably nearly 300 feet, but a space in the middle has 

 remained at its original height, so that a depression has been 

 produced like a great y, the portions both between and outside the 

 legs having remained standing. In the case of Thingvalla it appears 

 not unlikely that the lava which flowed down into the lake solidified 

 on coming in contact with the water, and formed a wall sufficiently 

 strong to hold up the lava plain till it formed a firm crust, and that 



