148 Mr Poulett Scrope's observations on the Volcanic 



embouchure in the valley of the Rhine, up to the foot of the 

 Feitsbers, one of the hills which form the circumference of 

 the lake of Laach ; from whence this, as well as many other 

 streams, (if they may be called so) of tufa, are derived. 



The basin of the lake of Laach is nearly circular and cra- 

 teriform, encircled by a ridge of gently sloping hills of no 

 great elevation. They are composed of irregular beds of loose 

 tufa, containing numerous fragments, and some very large 

 blocks of a variety of lava-rocks. Those which are most 

 abundant are of a basalt with very large and regular crystals 

 of black augite, and of olivine. Fragments also occur of tra- 

 chyte, sometimes of a whitish yellow colour and conchoidal 

 fracture; at others, of a coarse grain, consisting solely of crys- 

 tals of glassy felspar and hornblende. Some fragments are al- 

 so found similar to those which are common in the conglome- 

 rates of Somma, composed of an agglomeration of crystals of 

 mica, nepheline, meionite, Vesuvian, and other rare minerals. 

 No lava- rock appears in place within the interior of the basin, 

 and on its exterior, the only rock of this nature which shows 

 itself on the surface in the form of a regular current of lava, 

 is that in which the millstone quarries of Nieder-mennig are 

 worked. This stream certainly flowed from the crater of Laach, 

 the ridge of which suffers a depression on that side. The 

 eruption which produced it was probably the last, not only of 

 this particular vent, but perhaps of the whole district, as its 

 surface has an air of great freshness, and is not yet entirely 

 clothed with vegetation.* The rock of which it consists is ba- 

 saltic, with very few visible crystals of augite, and extremely 

 cellular, the cavities being very small and irregular. It is di- 

 vided into rude columns at the lower part of the current, 

 which is much more compact than the upper, but still cellu- 

 lar. It is here so hard as to be in great request for millstones, 

 which are exported to Holland in great numbers, and from 

 thence find their way to England. It envelopes numerous 

 fragments of quartz (always more or less vitrified and crack- 

 ed,) of granite, and other problematical rocks like those de- 



* This may have been the eruption recorded by Tacitus (xiii. lib. An- 

 nal.) as having ravaged the country of the Jutiones, near Cologne, in the 

 reign of Nero. 





