Davis.] 
346 
[January 18 , 
B. 4. Pit Crater Basins, Maare. Certain peculiar lakes in 
volcanic regions are characterized by a nearly circular form, and 
an abrupt depression of their basins below the surrounding gen- 
eral level ; the enclosing rocks are not always volcanic, and some- 
times there is scarcely a trace of materials that can have been 
ejected from the cavity : certain examples have a more or less 
complete wall of fragments around their rim, and in others this 
is quite wanting. Some observers take these to be the result of 
explosive eruption by which the ejected fragments were so finely 
broken and scattered over so large an area that they cannot now 
be identified; for evidence of this they point to the violent action 
known in some volcanoes, by which their form is greatly and 
quickly altered from a cone to a broadly open crater. 1 Others 
consider them the effect of subsidence either within an old crater- 
ring, or in the neighborhood of volcanic action (p. 333). It is 
difficult to make a choice here because decisive and critical obser- 
vations are wanting and are indeed as a rule unattainable; as in so 
many other cases, either cause is possible, but it is a very difficult 
matter to say which one really produced the known result. 
The most noted regions of recently extinct volcanic action in 
Europe — Auvergne and the EifeJ — furnish several examples of this 
species. In the latter, the Laacher See (i. e., Lake Lake) near 
the Rhine below Coblenz, is an oval cavity, a mile and a half in 
diameter and about two hundred feet deep; the surrounding sur- 
face is strewn with fragments from the hollow, but they do not 
form a distinct crater-wall enclosing it : other similar but smaller 
lakes, Pulver, Torf, Gemfinder and Weinfelder Maare, occur in 
the same region. 2 In Auvergne, Tazana, Pavin, and Chauvet may 
be named ; their rim is about at the level of the surrounding sur- 
face, and their walls show volcanic rock-layers that once apparently 
continued across their surface, still lying undisturbed around it: 3 
no lava flows can be traced from these cavities. 
1 Judd, Volcanoes, 1881, 174. 
2 BL v. Dechen, Geogn. Beschreibung der Vulkanreihe der Vorder-Eifel. Rheinland 
u.Westph. Ges. Verhandl., xviii, 1861, 1-190, p. 163 — . 
3 Scrope, Volcanos, 1862, 214- . Volcanos of Central France, 1858, 81, 143. 
A. v. Lasaulx, Ueber die Seen und kesselfdnnigen Wasserbecken im vulkanischen 
Gebiete Central- Frankreichs. Niederrein Ges. Sitz’b., xxv, 1868, 56, 67 ; Kratere und 
