Vol. 57.] 



AMONG THE MALVERN CAMBRIANS. 



175 



granular patches of sphene probably after ilmenite. The diabase 

 at its margin thus acquires an andesitic texture. It 

 differs from the olivine-bearing porphyritic rocks in several points, 

 notably in the very fine character of the groundmass. 



The diabases appear to have had a more marked contact-effect 

 upon the shales than had the olivine-basalts. The Grey Shales are 

 in many places baked yellow, or indurated into a rock resembling 

 hornstone (see p. 173). Spotted indurated shale may be observed in 

 debris near M 106. [For chemical analyses, see Table lY, p. 176.] 



Table III. 

 Olivine-Diabases which have been Microscopically examined. 



No. 

 103 



Loca lity. 



Texture. 



Specific Percentage 

 ! gravity. of Silica. 



South of Fowlet 

 Farm. 



Medium - grained : 

 ophitic. 



2-67 



106 ' Ou the road between [ Coarse - grained ; 

 I Eastnor and Holly- \ ophitic, 

 bush. ! 



2-75 



r 4.5-22 

 43-99 

 43-22 



182 { 



Coal Hill Cottage. 



Coarse-grained to 

 fine - grained ; 

 ophitic (to an- 

 desitic) 



2-71 

 2-51 



2-61 

 2-69 



184 i Southern end of 

 ! Coal Hill. 



Medium - grained ; 

 ophitic. 



2-64 



44-65 



372 



Close to Fowlet 

 Farm. 



Coarse - grained ; 

 ophitic. 



YII. Mutual Relations between the Rocks described. 



The porphyritic and the ophitic olivine-bearing rocks of the 

 district are mutually related, both in chemical aud mineralogical 

 composition and distribution. In each group the chemical com- 

 position appears to range from thoroughly basic to ultrabasic, and 

 the characters of the olivine and augite appear to have agreed in 

 the two series. Both series are developed chiefly in the southern 

 part of the district, but the diabases, unlike the porphyritic rocks, 

 are chiefly confined to a particular horizon in the Grey Shales. 

 The chief distinction lies in the generally coarser grain and the 

 early appearance of the felspar in the diabases ; these assume a 

 porphyritic form only at the margin, and then the type produced 

 differs from the ordinary olivine-basalts of the district. I have 

 been able to detect no transitional types between the two series. 



The amphibole-bearing rocks of andesitic type are quite distinct 

 from the olivine-bearing rocks. They differ from the porphyritic 

 members in their colour, chemical composition, and lower specific 



