746 GEOLOGY. 



stones. These are the straw, or caddis-worms, employed by the fisherman as a bait, really 

 the larvae of a tribe of four-winged insects, of which nothing is seen in the water but 

 the head and legs, by means of which they move, and drag along the case in which the 

 rest of the body is enclosed, and into which, on any alarm, they wholly retire. " The 

 construction of these habitations," says Kirby and Spence, "is very various. Some 

 select four or five pieces of the leaves of grass, which they glue together into a shapely 

 polygonal case ; others employ portions of the stems of rushes, placed side by side so as 

 to form an elegant fluted cylinder ; some arrange round them pieces of leaves like a 

 spirally-rolled riband ; others enclose themselves in a mass of the leaves of any aquatic 

 plants, united without regularity j and others, again, form their abode of minute pieces 

 of wood, either fresh or decayed. Other species construct houses which may be called 

 alive, forming them of the shells of various aquatic snails, of different kinds and sizes, 

 even while inhabited, all of which are immoveably fixed to it, and dragged about at its 

 pleasure a covering as singular as if a savage, instead of clothing himself with squirrels' 

 skins, should sew together into a coat the animals themselves. Even those that are 

 most careless about the nature of the materials of their houses, are solicitously attentive 

 to one circumstance respecting them, namely, their specific gravity. Not having the 

 power of swimming, but only of walking at the bottom of the water, by the aid of the 

 six legs attached to the fore part of the body, w^hieh is usually protruded out of the case, 

 and the insect itself being heavier than water, it is of great importance that its house 

 should be of a specific gravity so nearly that of the element in which it resides as, 

 while walking, neither to incommode it by its weight, nor by too great buoyancy ; and 

 it is as essential that it should be so equally ballasted in every part as to be readily 

 moveable in every position. Under these circumstances our caddis-worms evince their 

 proficiency in hydrostatics, selecting the most suitable substances, and, if the cell be too 

 heavy, glueing to it a bit of leaf or straw ; or, if too light, a piece of shell or gravel." In 

 a precisely similar way the cases which compose the indusial limestone we are noticing 

 are composed. Around the larva dwelling the shells of a small spiral univalve belonging 

 to the genus paludina, a tribe of fresh-water snails, are aggregated, and both the insects 

 and molluscs must have existed in countless swarms in the ancient lakes of central 

 France, since ten or twelve cases may be packed within the space of a cubic inch, and 

 some single strata of the indusial limestone are six feet in thickness, and may be traced 

 over an area of several miles. 



The district where these singular beds occur is the ancient Auvergne, so remarkable 

 for its long dormant volcanoes, the principal of which range in a chain from north to 

 south by the town of Clermont, partially shown in the annexed view. We have before 



remarked upon the abundant igneous 

 products of this region, immense 

 quantities of scoria? and lava occu 

 pying the surface, and the fact has 

 been referred to, that no notice of 

 volcanic activity has been preserved 

 by history or tradition, but that evi- 



Extinct Volcanoes of Auvergne. . , . ,, , 



deuce appears, in the cutting of deep 



channels by streams and rivers through the lava masses, of the vast remoteness of the era 

 when the crateriform hills poured them forth. An examination of the fresh-water 

 strata in the immediate vicinity of the volcanic rocks corroborates the conclusion. When 

 the ancient lakes first began to deposit their sediment, it may be inferred that the igneous 

 eruptions had not commenced. The absence of volcanic matter in the older tertiaries 

 warrants this assumption, for, had it then been scattered over the surface of the sur- 



