PALiGOZOOLOGY. 



81 



Rnd breccias, in the widest sense of these words, 

 reveal the character of a two-fold mode of ori- 

 gin. The materials of which they are mechan- 

 ically composed have not always been accumu- 

 lated by the waves of the sea, or by streams 

 of fresh water in motion ; there are fragment- 

 ary rocks in the production of which the shock 

 or the action of water has had no part. " When 

 basaltic islands, or trachytic mountains, arise 

 upon fissures, the friction of the rock as it as- 

 cends against the sides of the fissure causes ba- 

 salt and trachyte to become surrounded by ag- 

 glomerates of their own masses. The grains 

 of which the sandstones of many formations 

 consist have been more detached by the attri- 

 tion of outbreaking volcanic, or Plutonic rocks, 

 than produced by the motion of a neighbouring 

 ocean. The existence of such attrition-con- 

 glomerates, which are encountered in enormous 

 masses in both divisions of the globe, bear wit- 

 ness to the intensity of the force with which 

 the eruptive masses have been forced to the 

 surface from the interior. The waters then 

 obtained power over the smaller detached gran- 

 ules, and spread them out in layers which they 

 themselves covered"(^")- Sandstone forma- 

 tions are found intercalated among all the 

 strata, from the lower Silurian transition se- 

 ries, to this side the chalk in the tertiary for- 

 mation. On the edges of the vast plains of the 

 New World, both within and without the trop- 

 ics, they are seen as walls or bulwarks, indica- 

 ting, as it seems, the coasts against which the 

 mighty waves of a former ocean once dashed 

 themselves into foam. 



If we venture a glance at the geographical 

 distribution of rocks, and their relations in 

 point of place in that portion of the crust of the 

 earth which is accessible to our observation, 

 we perceive that the most generally distributed 

 chemical material of all is silicic acid, either in 

 the transparent and colourless state, or opaque 

 and variously tinged. Alter solid silicic acid 

 comes carbonate of lime ; then follow in order 

 the compounds of silicic acid with alumina, 

 with potash and soda, with lime, magnesia, and 

 oxide of iron. If the masses which we call 

 rocks be definite associations of a small number 

 of minerals, to which a few others, but also of 

 determinate kinds only, are added as parasites ; 

 if, in the eruptive rock granite, the association 

 of quartz (silicic acid), felspar and mica be the 

 essentials, so do these minerals, either isolated 

 or in pairs, present themselves in many other 

 strata. With a view of illustrating by an ex- 

 ample how quantitative relations distinguish a 

 felspathic rock from another abounding in mica, 

 I here remind the reader, as Mitscherlich has 

 done, that if three times more alumina, and 

 one third more silicic acid than belong to it 

 naturally be added to felspar, we have the com- 

 position of mica. Potash is contained in both, 

 a substance the existence of which in many 

 mineral masses reaches far beyond the com- 

 mencement of everything like vegetation on 

 the surface of the earth. 



The succession, and with this the age of the 

 several formations, are ascertained or deter- 

 mined by the reciprocal position of the Sedi- 

 mentary, Metamorphic, and Conglomerate stra- 

 ta, by the nature of the formations up to which 



the Eruptive masses ascend, but most certain- 

 ly and safely by the presence of organic re- 

 mains and the diversities of their structure. 

 The application of Botanical and Zoological 

 knowledge to the determination of the age of 

 rocks, the chronomctry of the crust of the earth, 

 which Hook's great spirit had already di- 

 vined(»«^), marks one of the most brilliant ep- 

 ochs in the progress of geology, now finally ab- 

 stracted, on the Continent at least, from Se- 

 mitic influences. Palaeontological studies have, 

 as with a vivifying breath, given grace and the 

 charms of variety to the doctrine of the solid 

 materials of the globe. 



The fossiliferous strata present us with the 

 entombed floras and faunas of bygone millen- 

 niums. We ascend in time, whilst, penetra- 

 ting downwards from layer to layer, we deter- 

 mine the relations in space of the several for- 

 mations. An animal and vegetable existence 

 that has passed away is brought to light. 

 Wide-spread revolutions of the globe, the up- 

 heaval of mighty mountain chains, whose rela- 

 tive ages we are in a condition to determine, 

 denote the destruction of old organic forms, 

 the appearance of new. A few of the older 

 still show themselves for a time among the 

 newer forms. In the narrowness of our knowl- 

 edge of original production, in the figurative 

 language with which this circumscription of 

 view is concealed, we designate as new crea- 

 tions the historical phenomena of change in the 

 organisms, as in the tenancy of the primeval 

 waters, and of the uplifted dry land. These 

 extinct organic forms are in one case pre- 

 served entire, even to the most minute details 

 of covering and articulated parts ; in other in- 

 stances nothing more remains of them than 

 their footsteps imprinted on the wet sand or 

 mud which they traversed when alive, or their 

 coprolites (petrified dejections), containing the 

 unassimilated portions of the food upon which 

 they fed. In the lower Jura formation (the 

 Lias of Lyme Regis), the preservation of the 

 ink-bag of the sepia(2«'), is so wonderfully per- 

 fect, that the same material which the animal 

 employed myriads of years before to preserve 

 itself from its enemies, has been made to serve 

 as the colour wherewith to paint its likeness. 

 In other strata there is sometimes nothing 

 more than the faint impression of a bivalve 

 shell, and yet will this suffice, when brought 

 by a traveller from a far distant country, if it 

 be a characteristic shell (Leitmuschel, a gui- 

 ding-shell) ("«) to inform us of the material for- 

 mations which there exist, and of the other or- 

 ganic remains with which it was associated ; it 

 tells the history of the district whence it came. 



The anatomical study of the ancient animal 

 and vegetable life of the globe extends in a 

 two-fold direction. The one is purely morpho- 

 logical in its bearings, and is especially devo- 

 ted to the description and physiology of the or- 

 ganisms ; it fills up with extinct forms the gaps 

 encountered in the series that still exist. The 

 other direction is geological, and considers fos- 

 sil organic remains in their relations to the su- 

 perposition and relative age of the sedimentary- 

 formations. The first was long the usual di- 

 rection taken, and in its imperfect and superfi- 

 cial comparisons of petrifactions with living spe- 

 cies led off into erroneous ways, traces of which 



