234 Murchison s Silurian System. 



rally highly inclined at their junction with Silurian rocks, in- 

 dicating a period of great physical changes to have elapsed 

 between the time they were consolidated and the commence- 

 ment of Silurian deposits. 



Trap and altered Silurian Rocks. — Great outbursts of 

 trap rocks took place during and after the accumulation of 

 Silurian rocks. Mr. Murchison describes tracts of Caradoc 

 sandstone cut and thrown up into vertical beds in a state 

 approaching to quartz rock, being much indurated, and in 

 parts cellular. Where the sandstones come in contact with 

 trap, it is only to be detected in the state of true quartz ; in 

 most instances, the traces of bedding being quite destroyed. 

 Trap rocks when erupted in contact with schist, convert it into 

 an indurated form, or Lydian stone ; of this there are several 

 instances, vide Murch. p. 320, where the schist has been origi- 

 nally perhaps of bituminous character, as shale ; anthracite 

 is not an uncommon mineral in the altered rock. Mr. Mur- 

 chison details an instructive instance of this nature, where a 

 credulous farmer ruined himself by mistaking one of these 

 indications of anthracite in the lower Silurian rocks for coal. 



Greenstone often contains fragments of the rocks through 

 which it is projected in a state more or less altered; 

 sometimes black crystals of carbonate of lime, iron py- 

 rites, and little flakes of anthracite and mineral waters ; when 

 these issue from contiguous springs in volcanic districts, they 

 are often very different in their nature, but when issuing 

 from greenstone they are generally sulphureous and saline. 



The older Silurian rocks are covered, in certain localities 

 described by Mr. Murchison, with stratified sedimentary 

 rocks, alternating with nodular concretionary deposits, which 

 he regards as volcanic grit, proving that repeated igneous 

 action took place during the period when the upper Silu- 

 rian strata were accumulating. 



In some instances stratified trap rocks, consisting of con- 

 cretions of compact felspar, containing crystals of common 



