172 J. A. PHILLIPS ON THE SO-CALLED 



It will be observed that its composition very nearly agrees with 

 that of the crystalline hornblendic slates in the neighbourhood of 

 Penzance. 



A similar rock occurs on the headland forming the northern 

 boundary of the Cove, and is associated with a brecciated qnartzite, 

 the angular fragments of which have been cemented together by 

 crystalline hornblende. 



Prom the foregoing description of the rocks of this district it will 

 be seen that they are in all respects almost identical with those in 

 the neighbourhood of Penzance. Nineteen sections of rocks from 

 this part of the coast were cut and examined. 



St.-Ives District. — The next considerable exposure of rocks be- 

 longing to this class is that laid down on the Government Geological 

 Map as extending, with but one inconsiderable break, from a short 

 distance east of Trevail Point to a little beyond Porthminster Point, 

 in St.-Ives Bay ; its total length is three miles, and its greatest 

 width about a mile. The only interval in the continuity of 

 these rocks occurs along the valley through which passes the turn- 

 pike-road leading from St. Ives to Halsetown and Towednack ; this 

 has been formed by the erosion of clay-slates which here come 

 to the surface. The rocks in this neighbourhood are very similar in 

 character to those forming the shores of Mount's Bay, and consist of 

 hornblendic slates, interbedded with augite rocks and occasional 

 bands of clay-slate. 



Although numerous thin sections were prepared from the rocks 

 of this district, they are all so nearly identical in their petrological 

 characteristics with those from the neighbourhood of Penzance, that 

 it has not been thought desirable to subject any of them to analysis, 

 and a detailed description of their appearance under the microscope 

 becomes unnecessary. It may, however, be stated, generally, that 

 a large outcrop of crystalline rock, very similar to that of Tolcarn, 

 occurs in the immediate vicinity of St.-Ives pier. At Clodgy Point 

 the rock contains well-defined and unaltered crystals of plagioclase, 

 slightly altered augite or diallage, with pseudomorphs after one or 

 other of these minerals, and pseudomorphic crystals after magnetite ; 

 it also contains viridite, a little granular quartz, and a few crystals 

 of apatite. 



Near Carrack Olu there are large banded masses of very hard and 

 tough greenish slates, consisting of reticulated crystals of hornblende, 

 with a little viridite and quartz, disseminated throughout a subor- 

 dinate amount of apparently felspathic base. Near Porthminster 

 Point, at the eastern extremity of the formation, ordinary clay-slate 

 alternates with more or less crystalline rocks. The slate, which is 

 here much contorted, is hardened by metamorphic action, and is 

 traversed by numerous veins of quartz ; quartz also frequently 

 occurs in thin layers following the lines of cleavage ; thin sections 

 of these slates present the usual characteristics of ordinary killas. 

 Twelve sections were prepared from these rocks. 



District between St. Erth and Camborne. — On referring to the 

 Geological Map, numerous bands of greenstone will be observed ex- 



