Vol. 57.] AMOXG THE MALVEEN CAMBRIANS. 181 



in a certain number of instances the basalt has bleached, not only the 

 Shales below, bat also those above, a circumstance which proves that 

 at least some of the sheets are sills. Bat perhaps the strongest 

 piece of evidence for the intrasive nature of the basalt-sheets is the 

 complete absence of tuifs or, indeed, of pyroclastic materials in 

 the Cambrian sediments. The available evidence points, then, to 

 Prof. Lapworth's conclusion that of the igneous rocks in question 

 ' the majority are certainly intrusive ' (see p. 157). We may note, 

 as in agreement with this conclusion, that the igneous rocks 

 associated with the Cambrian of other parts of the Midland district 

 are intrusive, and that, as pointed out to me by Prof. Watts, 

 Cambrian volcanic rocks nearer than South Wales are not known. 



XI. Date op Intrusion. 



The rocks described in the foregoing pages have apparently 

 shared the foldings and dislocations which affected all the older rocks 

 of the Malvern district. I maintained on a former occasion ^ that 

 these movements dated principally from Coal-Measure times. The 

 olivine-diabases which have invaded the Carboniferous of the 

 English Midlands possibly date in part from this period, for 

 they seem to have invaded the Lower and Middle Coal-Measures, 

 but not the Upper. The igneous rocks intrusive in the Malvern 

 Cambrians bear the stamp of greater antiquity. Hocks which, to 

 the eye seem fresh and un weathered, show under the microscope 

 considerable physical and chemical changes. The olivine-diabases 

 are never so well ])reserved as the very similar Carboniferous 

 diabases. The felspars of these and the olivine-basalts so com- 

 monly show undulatory polarization that the determination of the 

 species is generally a matter of difficulty. The olivine is invariably 

 serpentiuized, and the serpentine has given rise to tertiary minerals 

 such as biotite." If it be granted that the olivine-diabases and 

 basalts and the andesitic camptonites are intrusive, it follows that 

 the intrusion took place, at any rate in part, in late Tremadoc or in 

 post-Tremadoc times, for all three types invade the uppermost 

 Tremadoc Shales seen. On the other hand, as Phillips pointed out, 

 the igneous rocks do not invade the May Hill Sandstone.^ It 

 follows, therefore, with high probability that injection took place in 

 Ordovician times, or at least not before the deposition of the upper- 

 most Tremadoc Beds, and not later than that of the lowest May 

 Hill Beds in the district. This is in agreement with the fact that 

 the allied rocks of other districts are not known to invade any rocks 

 later than the Cambrian. 



^ Quarh. Joiirn. Geol. Soc. vol, Ivi (1900) pp. 175 et seqq. 



^ It must be noted, however, tliat the imperfect preservation of the Malvern 

 rocks is doubtless due in part to the absence of deep or recently-worked 

 quarries. 



3 Mem. Geol. Surv. Gt. Brit. vol. ii, pt. i (1848) p. 57. 



