52 RAMBLES AND REVERIES. 



in the upper Cambrian strata. It will be seen then 

 that graptolites furnish a most valuable test of the 

 age of the rocks in which they occur. For want 

 of a little information of this kind a speculator 

 some years ago spent a large sum in searching 

 for coal amongst the black shales of Tullygirvan 



FIG. %.Didymograptw Murchisoni. 



in Ireland, notwithstanding that every blow of the 

 pick revealed quantities of graptolites which would 

 have informed the merest tyro in geology that he 

 was working rocks that had been deposited vast 

 ages before the coal forests began to grow. 



The best places in this country for obtaining 

 these most interesting fossils are the Lower Silurian 

 beds of Dumfriesshire, the soft shales of which are 



