158 PROF. T. T. GROOM ON THE IGNEOUS ROCKS [Feb. I9OI, 



diabases and basalts,^ and Prof. Watts ^ compared certain of the 

 latter with 'diorites' associated with the Cambrian strata of other 

 parts of Great Britain. 



It will be seen from the foregoing summary that the igneous 

 rocks in question have been, with few exceptions, regarded as due 

 to contemporaneous volcanic action, supposed to have taken place 

 either in Cambrian times or soon after. 



II. Introdtjction. 



The general distribution of the igneous rocks in the Cambrian 

 strata of Malvern has been indicated in a former communication ^ ; 

 it will be sufficient, therefore, on the present occasion to point out 

 that thev occur at all horizons, and at all distances from the 

 Archaean* axis, though they are s[)ecially abundant at certain levels 

 and in certain localities. They are found in the form of sills, 

 small laccolites, bosses, or dykes. The preservation of the material 

 leaves much to be desired, and it is only by having a large number 

 of rocks sliced that it has been found possible to reduce the series to 

 a few relatively constant types. I was formerly disposed to regard 

 the more acid porphyritic members as amphibole-basalts (and 

 andesites) of a peculiar type*; but a renewed investigation with 

 the aid of further material has led me to doubt the propriety 

 of these designations, and to regard the prevailing rocks of the 

 district as ophitic olivine-diabases ; olivine-basalts of 

 somewhat unusual type; and amphibole-bearing porphyritic 

 rocks of andesitic habit, but probably related, or belonging, to 

 the camptonites. 



III. Amphihole-beaeing Eocks of Andesitic Habit. 

 (PI. VII, figs. 5 & 6, & text-figs. 2, 5, pp. 161, 165.) 



The igneous rocks prevailing in the Hollybush Sandstone are of a 

 somewhat peculiar type. With two exceptions (M 236 & M 1176)^ 

 all the intrusive masses in the Sandstone belong to this type. 

 Similar rocks also occur in both the Black and the Grey Cambrian 

 Shales. The type is characterized by the presence of needles and 

 stout prisms of a species of aniphibole, often accompanied by pheno- 

 crvsts of augite and felspar, and sometimes, perhaps, of ilmenite. 

 These are set in a fine-grained andesitic groundmass, the felspar- 

 laths of which very generally exhibit fiow-structure. According to 

 Timins's analyses (see Table IV, p. 176) the percentage of silica 



1 Quart. Journ. Geol. See vol. Iv (1899) pp. 131, 141, 158 et seqq., 164 

 et seqq., and Rep. British Assoc. 1900 (Hradfoid). 



2 Quart. Jouni. Geol. Soc vol. Iv (1899) p. 169. 



3 Ibid. pp. 133, 138, 141. 157 et seqq. 



4 Rep. Brit. Assoc. 1900 (Eradrord). 



5 Tlirouo'hout this paper the numerals in parentheses preceded by M refer 

 to numbers in tlie writer's miip, Quart. Journ. Geol. Soc. vol. Iv (1899) pi. xiii, 

 and to numbered specimens in his collection. 



