356 MINERALOGY OF THE REOHEAD. 



In these globular concretions, which are much 

 harder than the surrounding rock, different species 

 of the shells termed borers, form their retreats. 

 I observed the following : — Mytilus rugosus, Mya 

 suborbiadariSf Venus perforans, and Venus yuU 

 lastra of the Testacea Britannica of Montagu. 



2. — Sandstone-Conglomerate. The rock to 

 which this name has been applied, consists of por- 

 tions of the older rocks of granite, gneiss, mica- 

 slate, quartz, porphyry, and hornblende. These are 

 in the form of rounded balls, or blunt angular frag- 

 ments, seldom above six inches in diameter. The 

 interstices between the balls, are filled up with 

 smaller fragments of similar materials, and the 

 rock possesses a considerable degree of cohe- 

 sion. In this rock, there are numerous narrow 

 lines of sandstone, sometimes also thin beds 

 parallel with the surface of the stratum which 

 encloses them. Similar appearances are observ* 

 able in the hills of gravel. 



This conglomerate, or gravelstone, occurs in 

 two different situations. In the first, it alter- 

 nates in beds with the sandstone, as we have 

 already mentioned ; and in the second, it occu- 

 pies an unconformable position in regard to the 

 sandstone. At the western extremity of the 

 high cliffs, which terminate about a mile to 

 the eastward of Aberbrothick, the strata of sand- 

 stone rise with an inclination of IS**. The ex- 

 tremities of these strata, are covered with a 



