236 



Murchison s Silurian System. 



by the stratum charged with Terebratula navicula, and 

 underlaid by the Pendle beds (d) of the lower Ludlow rock. 



The locality in which this fault occurs in the Silurian rocks, 

 is situated between two great axes of volcanic outburst, the 

 Dee Hills and the Caradoc, to which this and other similar 

 dislocations in the same district may be ascribed. 



One of the effects of sudden outbursts of volcanic rocks, 

 amidst consolidated strata, is to separate the latter ; if this 

 be at the same time accompanied with great elevatory move- 

 ments, large masses of the disturbed strata are liable to give 

 way and become detached. These Mr. Murchison calls 

 outliers. It is always necessary to be able to distinguish 

 outliers, since they are often associated with formations to 

 which they do not belong. Their investigation also, is cal- 

 culated to throw much light on the circumstances under 

 which the causes which produced them took place. Three 

 outliers of the Ludlow rocks are described by Mr. Murchi- 

 son ; one of these, called Tinker's Hill, rises on the south- 

 west bank of the Teme. It consists chiefly of small nodules 

 imbedded in sandy calcareous shale, occasionally united in- 

 to flag-like beds of bluish grey earthy limestone, containing 

 Terebratula Wilsoni^ and other characteristic fossils of the 

 lower Ludlow rocks. This outlier is two miles in length, and 

 is parallel to the main direction of the Silurian deposits. 



The direction of the joints and fractures of strata is also 

 an important point, on which subject the reader is referred 

 to Phillip's Geology. In some chains, Mr. Murchison re- 

 marks there are long fissures, either coincident with the 

 line of elevation, or nearly at right angles in the direction of 

 the dip. These may frequently, Mr. M. remarks, be ascribed 

 to the elevation of strata en masse; but must not be con- 

 founded with the symmetrical lines by which beds of rock are 

 often intersected, and which are often the greatest conveni- 

 ence in quarries ; for where these straight chinks cut the 

 rocks across their bedding, there is little more to be done 



