Button — Classification of Rocks. 503 



into existence, as far as we yet know, in Cambrian times, BracMo- 

 poda abounded in Silurian seas, but the class attained its maximum 

 development in the Carboniferous period. A small number of 

 species have been taken from Permian rocks, but Triassic strata have 

 not hitherto yielded us any. When we examine Liassic and Oolitic 

 strata, we find again a large number of species, which, however, 

 become fewer as we ascend the scale, until we reach the Portland 

 rocks, in which no Brachiopod has been discovered. The class again 

 increases in importance in Cretaceous strata, and again diminishes 

 in Tertiary formations, which have hitherto furnished us with not 

 more than eight or nine species. Of living Brachiopoda seventy 

 species have been described ; these have a wide geographical range, 

 and have been found both in littoral waters and in seas of great depth. 

 Although in British seas Brachiopods are very rare, yet they are 

 by no means so in the seas of Southern latitudes, the bays and 

 harbours of Australia swarming with Waldheimia and other forms of 

 this interesting and remarkable class of the animal kingdom. 



N 



V. — On the Classification of Books. 



By Capt. F. W. Hutton, F.G.S. 



one, I think, will deny that geology is far behind all the other 

 sciences in the classification of those substances which form its 

 special study, viz., rocks. No two authors agree on the subject, and 

 no one seems to have attempted to form a scientific classification, 

 based on a natural system. 



The reason perhaps is, that nearly all the classifications we have 

 are from the hands of chemists and mineralogists rather than geolo- 

 gists ; and no geologist, therefore, can admit their systems as natural, 

 or as adapted to his purpose. How, for instance, could a geologist 

 think of classifying an aphanite-slate, interbedded with quartzite and 

 clay-slate, with an aphanite dyke cutting across the rocks in an 

 uncertain direction ? It is absolutely essential for him to determine 

 to which class the piece of aphanite he may just have broken ofi'with 

 his hammer belongs, as it will make a vast diff'erence in the structure 

 of the country he is examining, and in the geological map of the 

 district. Cotta, indeed, says that " we cannot lay do^m a logically 

 confplete system of classification, to embrace all rocks, on any 

 principle." In this I quite agree with him ; but any one principle 

 means an artificial classification, and I believe that it is quite possible 

 to form a natural system complete enough to answer all the wants of 

 a geologist, and I hope that I shall not be considered presumptuous 

 for having, with this view, drawn out the following table, in which 

 I have tried to make the rocks fall into as natural groups as possible. 

 That I have not succeeded in all cases I am the first to admit, but I 

 hope that it will not be without its use in showing what appears to 

 me to be the most natural system that we can adopt at present. 



In drawing up this table I have tried to base the different divisions, 

 etc., on facts well established, and not on debated theoretical points. 

 This has led me to reject, after much consideration, some names in 

 common use, such as '' Igneous," "Intrusive," etc., and to substitute 



