4 Essay introductory to Geology. 



To express these shortly — the lowest, and, as they are called, primitive 

 formations, (granite, gneiss, micaceous, and other primitive slates, primitive 

 marble, &c.) are well known to be destitute of organic remains, which first 

 appear in the next, or transition class, (consisting of coarse roofing slates 

 and various sandstones, called by the Germans grauwacke, together with 

 interposed beds of limestone). With these it is clear that the carboniferous 

 series of rocks must, from the close generic relation of their organic re- 

 mains, both animal and vegetable, be associated as the upper groupe of the 

 same order. The anthracite, or culm alternating with the roofing slates 

 of North Devon, is in fact a true grauwacke coal formation, and closely 

 agrees in the relics of vegetables which it exhibits, with the great coal 

 formation. The remains of this most ancient groupe, which thus exhibits 

 the first traces of animal life, are very numerous species of corals and 

 madrepores ; specifically distinct, but not perhaps generically so widely 

 separated from the existing types of this class, as is the case with most of 

 the other divisions of the animal kingdom here imbedded. 



Of the echinodermata, the crinoidea present six genera, including eighteen 

 species ; all of these belong to the division inarticulata, characterised by 

 having the part enveloping the viscera inclosed by thin testaceous plates, 

 something hke the shell of the echinus, while all the species in the rocks 

 younger than this lowest order, agree with the only species of this once 

 very numerous class still known to exist, in having the lateral portions of 

 the frame work, containing the soft parts, formed into more thick and 

 regularly articulated ossicula. Here we see the geological and natural 

 subdivisions of this interesting animal class mutually illustrating each 

 other. This family of echinodermata was first fully described, and all the 

 complicated details of its anatomy minutely explained, by the sagacity and 

 admirable industry of Mr. Miller, the late Curator of the Bristol Philo- 

 sophical Institution, in his unrivalled monograph of the crinoidea. His 

 extensive collections for this purpose form a most valuable portion of the 

 museum there established. 



Of the bivalve mollusca nine-tenths belong to the single family of bra- 

 chiopoda, which is distinguished by having a pair of ciliated spiral arms, 

 one on each side the mouth, and forming an attachment to the bodies, on 

 which it rests by a pedicle issuing from a perforation over the hinge of the 

 convex valve. In the fossil genera the adaptation of the shells to the pecu- 

 liar form of this class of mollusca is clearly seen, and in one instance spiral 

 calcareous loops are preserved in the interior of the fossil shell, (hence 

 called spirifera), which appear to have formed a support to the arms, like 

 the elastic calcareous loop in the recent terebratula. The fossil genera of 

 the transition series are however widely different for the most part from the 

 recent, and often assume what to eyes acquainted with recent Conchology 

 must appear very grotesque forms. There are several of these extinct genera, 

 some confined to the lowest, others common to the lias, and other more recent 



