122 



GEOLOGY OF THE BRISTOL COALFIELD. 



are those of the Tortworth district, and were ejected at a very early- 

 period of the earth's history,— after the Silurian, and before the 

 Carboniferous era had fairly commenced. Tbis is plainly shewn 

 in the following sections at Horsley quarry and Damory Biidge, 

 especially the latter place. Of course these sections are only 

 diagrammatic, and not drawn to scale. 



Tig 1. 



a Igneous rock, b Upper Llandovery Sandstone. 



e Lias. 



c Devonian, d Trias. 



Fig. 2. 



a Igneous rock, b Upper Llandovery Sandstone c Altered Llandovery, 

 d Upper Silurian, e Devonian. / Trias ff Lias. 



The beds of the Upper Llandovery, through which the Green- 

 stone passed, were lifted up and partially altered. In the parts 

 nearest the surface, which allowed the steam and gases to escape, 

 the rock still shows the impressions of the bubbles and is 

 vesicular in its character, and where they were large and 

 afterwards filled with earthy matter, forms the kind of stone 

 termed by mineralogists. Amygdaloid. At the quarry near 

 Damory Bridge, the sands may be traced from loose strata 

 containing the usual fossils through several transition stages, as 

 semi- molten sand to those parts actually in contact with the hot 

 lava when transparent and solid quartz rock became the product. 



