229 



Thuringia, and the Hartz, they present the same palaeontological, 

 stratigraphical and petrographical characteristics. The chain acquired 

 individuality towards the close of the Palaeozoic epoch in consequence 

 of the extensive crumpling of the Palseozoic sediments and their associ- 

 ated igneous rocks. The final relief of the intense stresses which 

 produced this crumpling appears to have been afforded by the intrusion 

 of granite at many points along the chain. 



The igneous rocks associated with the Devonian and pre-Devonian 

 sediments are partly intrusive and partly contemporaneous (lavas and 

 tuffs). They present essentially the same characters wherever they occur. 

 The rocks of the Hartz and the Fichtelgebirge are often undistinguish- 

 able either in respect of their original or secondary characters from 

 those of Devon and Cornwall. It is an unfortunate circumstance that 

 the exact boundary which separates the Lower Palaeozoic from the 

 Devonian strata has not yet been traced in the west of England, and 

 we are therefore in doubt as to the precise horizons at which some of 

 the igneous rocks occur. The difficulty experienced in tracing this 

 boundary is undoubtedly to be attributed in great measure to the power- 

 ful character of the post-Carboniferous earth-movements. These have so 

 masked the original junctions, modified the strike of the earlier rocks 

 and metamorphosed the sediments that what appears now as a geological 

 unit may after all be a complex of different formations. For our present 

 purpose it will be convenient to separate the basic rocks of the Lizard 

 District from those of the rest of Cornwall and Devon. The latter are 

 associated with Devono- Carboniferous and Lower Palaeozoic strata, and 

 they have been metamorphosed in early Carboniferous or post-Carboni- 

 ferous times ; the former are associated with gabbro and serpentine 

 and the period of their metamorphism has not yet been absolutely 

 determined. 



The basic igneous rocks, associated with the Paleozoic sediments, occur 

 as lavas and tuffs, and also as sheets and dykes. The unquestionable lavas 

 and tuffs are found in Devon and East Cornwall. They are associated with 

 Devonian strata. In West Cornwall there are certain green hornblendic 

 rocks which may be in part metamorphosed tuffs. Dykes and veins of 

 greenstone are rare, except in the Lizard District/ 1 ) 



We are principally indebted to Messrs. ALLPORT/ 2 > PHILLIPS, (S) 

 RuTLEY, (4) and WORTH (5) for the information we possess as to the petro- 

 graphical characters of the " greenstones " of the West of England. The 

 description of these rocks is attended with considerable difficulty in con- 

 sequence of variations in structure and composition in one and the same 



(1) The dykes of mica-trap will be described in a subsequent chapter. Some of these, as 

 for example those of To wan Head, near New Quay, closely resemble greenstone. They are, 

 however, of later date than the rocks now under consideration. 



(2) Q. J. G. S., Vol. XXXII., 1876, p. 407. 



(3) Q. J. G. S., Vol. XXXII., 1876, p. 155, and Vol. XXXIV., 1878, p 471. 



(4) The Eruptive Rocks of Brent Tor and its Neighbourhood. Mem. Geo. Survey, 1878 ; 

 Q. J. G. S., Vol. XXXVI., 1880, p. 285, and Q. J. G. S., Vol. XL1L, 1886, p. 392. 



(o) The Rocks of Plymouth. Trans. Plymouth lust. &c., 1886. 



