W. A. E. Ussher — Post- Tertiary Geology of Cornwall. 203 



In short, as it seems to me, the classification of rocks must not be 

 attempted by relying on evidence obtained in the field, with the micro- 

 scope, or from the laboratory alone. Sometimes, it is true, the first 

 will teach all that it is really important to know; commonly, however, 

 we shall have to appeal to the second source of information, and not 

 seldom we shall obtain from the third the solution of a difficulty 

 which the other methods have failed to answer ; but this last is 

 rather a court of appeal than a court of first instance, and one which 

 will not so much reverse the judgments of its predecessor as decide 

 points which had been of necessity reserved. Or, to put it other- 

 wise, and perhaps more accurately, the chemist is called in, like an 

 expert, to settle certain questions, but there are many others with 

 which he is not competent to deal. Professor Dana, as I venture 

 to think, notwithstanding his own extensive learning as a geologist, 

 places too much reliance on chemical evidence ; and thus, while 

 aiding in the destruction of certain idols which have a detrimental 

 effect in Science, has set up others in their stead which would be 

 as hurtful to its progress. 



V. — Post-Tertiary Geology of Cornwall. 1 



By W. A. E. Ussher, F.G.S. 



(Part III. — Continued.) 



The Eaised Beaches and Associated Deposits op the Cornish 



Coast. 

 15. St. Ives. 



a. On the east of Carrack Olu Point, a bed of pebbles, 1 foot thick, 

 is shown under Head, at from 2 to 5 feet above high water. The 

 greenstone composing the Point is capped by a Head of yellowish- 

 brown loam with angular fragments of greenstone. 



b. In the bay east of the above, near the north part of St. Ives, 

 the section is as follows : — 



Head, with large angular fragments 5ft. Oin. 



Impersistent strip of yellowish -brown loam. 



Head, loam with a few subangular fragments, and boulders 



toward the base 4ft. Oin. 



Olive and yellowish sand with occasional pebbles 10ft. Oin. 



At base about 5 feet above high water ; resting upon dark bluish 

 slaty grit with numerous joints. 



c. On the north part of St. Ives Island, the greenstone is capped 

 by a Head of angular greenstone fragments from 10 to 15 feet in 

 thickness. 



d. Mr. "Whitley (Journ. E. Inst. Corn. No. 11, p. 184) gives the 

 following section of the raised beach in Porthgwidden Cove, St. Ives : 



Greenstone soil, upon Head of large angular blocks of hornblendic \ • . ' . 



rock. Pine sand and loam ; upon pebbles of hornblendic rock, I „q » . 



quartz, granite, and a few worn flints, mixed with sand, and I +i,ick 



containing layers of fine brown sand J 



The base of the deposit is given as 5 feet above high water. 

 1 Continued from the April Number, p. 172. 



