22 Rev. W. D. Conybeare on the Phenomena of Geology. 



cent. We also find that effects analogous to those which 

 characterize the earlier rocks may be produced by igneous 

 action. 



Observations. All rocks may be conveniently classed under 

 the comprehensive genera Calcareous, Quartzose, and Argilla- 

 ceous. We may examine what have been called the formation 

 suites of each of these in order. I. The Calcareous class pre- 

 sents, 1st, in the earliest deposits saccharine marble; 2ndly, 

 compact and semi-crystalline rocks in the transition and car- 

 boniferous series ; Srdly, rocks of less compact and looser 

 texture in the oolites ; ^thly, earthy rocks in the chalk and 

 tertiary formations. II. The Quartzose series exhibits, 1st, 

 crystalline quartz rock; 2ndly, compact sandstones in the 

 carboniferous formation ; Srdly, looser sandstone ; 4thly, sand. 

 III. The Argillaceous series is represented, 1st, by com- 

 pact clay slate in the lowest deposits ; 2ndly, by semi-indu- 

 rated shales in the carboniferous group ; and Srdly, by com- 

 mon clay in the subsequent formations. 



Now these changes are analogous to those which are 

 known, or generally believed to be the result of igneous ac- 

 tion. In Sir J. Hall's experiments on the fusion of lime under 

 the pressure of a column of water, crystalline marble was pro- 

 duced : and besides these actual experiments we may refer to 

 the changes effected by trap dykes, as universally acknow- 

 ledged to be of igneous origin. In the north of Ireland the 

 chalk where covered by trap becomes a compact limestone, 

 and where intersected by trap dykes, assumes completely the 

 texture of primitive saccharine marble for some yards from 

 the contact. The lias shales become here and in Scotland 

 altered by the contact with trap into flinty slates, and loose 

 sandstones into compact and crystalline. In Professor Hens- 

 low's very valuable account of Anglesea, in the Cambridge 

 Philosophical Transactions, much information on this subject 

 will be found. In one place he describes a mass of granitic 

 texture which appears to have resulted from sandstone thus 

 altered. In Cornwall and the Lead Hills of Scotland, wherever 

 the granite protrudes through the incumbent grauwacke, we 

 find an intervening zone approximating in its characters to 

 gneiss, which certainly appears to be grauwacke altered by the 

 contact : and Bone believes this to be the case generally with 

 the gneiss and mica slate of the Pyrenees. 



The Alps appear at first sight to present an exception to 

 the general rule announced at the head of this article; but it is 

 in truth an exception of that kind which proves the rule. Here 

 the limestones contemporaneous with our oolites still preserve 

 a highly compact and crystalline character. This is well de- 

 scribed 



