128 Dr. Berger o;/ the physical Structure 



menaclianlte by the the Rev. Wm. Gregor (tlie Titane oxyde fer- 

 r'lfere of Haiiy) is found \\\ a stream work. 



Before arriving at Port-hallo, I observed in the village of Tre- 

 gollis, in the parish of St. Kevern, a bed of greenstone of small 

 extent, and this is the last place in Cornwall where I observed that 

 subordinate rock of the grauwacke formation. According to the 

 geological rather than specific characters, this greenstone ought to 

 be considered as belonging to the transition rocks.* 



The next formation in importance, which succeeds the grau- 

 wacke, is the serpentine. But although it may be seen for some 

 miles before, in the form of large blocks scattered here and there 

 in the fields, and forming part of the materials of the walls of the 

 inclosures, it is not upon this rock that the grauwacke immediately 

 rests, but on a kind of talcose slate, of a greenish grey colour at the 

 surface, tender and shining, soft to the touch, and evidently stratified: 

 it forms a bed of small thickness, the boundaries of which it is dif- 

 ficult to determine. The junction may be seen in the cliffs to the 

 S.S.E. of Port-hallo, immediately upon crossing the brook which 

 runs through that village in its passage to the sea. 



A few paces beyond this bed of talcose slate, the serpentine ap- 

 pears in the form of large blocks of an irregular shape, detached one 

 from another and projecting above the surface of the ground : these 

 soon disappear, and on the way to St. Kevern's Church, on the 

 S.S.W. we enter again, for a short time, upon the grauwacke, which 

 shews that these rocks are included in an irregular manner one 

 within the other ; and in a cultivated country it is not easy to trace 

 the line of demarcation. It is quite clear, however, that to the S. 



* Secondary as well as primitive greenstone is composed of hornblende and fi Ispar, but 

 the grains of it are less crystalline, and less intimately blended. Brocliant, Traite de 

 Miueralogie, tome ii. p, 60. 



