THE GEOLOGICAL STRUOTtTRE OF SOUTH CORNWALL. 263 



or called in question, until the paper with which we are dealing 

 made its appearance in your Journal, in August, 1881. 



Mr. Collins differs very widely from the conclusions of all 

 previous observers, in breaking up the Strata into no fewer than 

 four well-defined formations, viz., the Cambrian, Lower Silurian, 

 Upper Silurian, and Devonian, all of which he claims to have 

 well marked lithological and palaeontological characteristics, and 

 also to be separated from each other by lines of unconform- 

 ability, which are to be seen in many parts of the county under 

 consideration. 



The distributions of these various formations as laid down 

 on the map which accompanies Mr. Collins' memoir, presents a 

 most intricate and network-like appearance, which if it really 

 exists as delineated on the map, reflects the very highest credit on 

 his powers as an observer in unravelling the wonderfully 

 complex interweaving of the different formations represented. 



Taking the different formations into which he has divided 

 the strata in their regular order, we will begin with the lowest 

 or oldest. 



Pre-Silurian or Cambrian.— y^e find this occupies a large 

 space on his map, covering nearly the entire area on the north 

 coast between St. Agnes Head and G-odrevy. On the south 

 coast it extends from near Calenick to beyond Penryn, in the 

 direction of Falmouth, and also occurs at a few other localities 

 in the neighbourhood, whichmay be seen by referring to the 

 map. 



The character of the rocks which make up this formation 

 is described as containing neither sandstone nor limestone, and 

 as being entirely devoid of organic remains. " They are " he 

 says (to use his own words) "all extremely ancient in appear 

 ance, generally very siliceous and rough to the touch, except 

 where they are black and carbonaceous, or where scales of mica 

 or talc have been developed between thin laminae. " 



He also mentions that " a band of what appears to be 

 true gneiss, nearly half-a-mile in width, has also been developed 

 in these Pre-Silurian rocks near Penryn, close to the junction of 

 the killas and granite." 



To the general aspect of these so-called Cambrian rocks as 

 above described we will not demur, but the occurrence of a true 



