part 2] TERTIARY OUTLIERS OF THE WEST OF ENGLAND. 211 



and Cornwall, but absent (so far as my observation goes) x in the 

 Greensand of the Haldons, are amphiboles (hornblende, actinolite), 

 pyroxenes, biotite, chlorite, and epidote. 



In summarizing the petrological characters of the Greensand of 

 the district, it may be said that — 



(1) The heavy detrital minerals are coarse in grain and abundant in 



quantity. 



(2) No important and systematic variation in quantity or character of the 



residue is noted, either laterally over the small area covered, or 

 vertically through the very different lithological divisions. 



(3) Tourmaline of all tints, shapes, and sizes is by far the commonest 



constituent. 



(4) Muscovite, and occasionally glauconite, is very abundant, occurring in 



large grains. 



(5) Andalusite (mainly pleochroic) and staurolite are exceedingly plentiful. 



Topaz is rather less abundant. 



(6) Kyanite occurs, but is rather irregular in its distribution. It is in 



some cases very abundant ; in others it is rare. 



(7) Garnet is extremely rare or absent. 



(8) Magnetite is absent, and ilmenite often rare. 



(9) Amphiboles, pyroxenes, biotite, and chlorite appear to be absent. 



Of the minerals enumerated above, the subordinate importance 

 of magnetite is general in sediments, and possibly also in many 

 igneous rocks. 2 The rarity of ilmenite is noticed again on p. 223. 

 The rarit} r of garnets, a feature which the Cretaceous System of the 

 West Country shares with the Eocene, Oligocene, and Pliocene (see 

 later, p. 221), is noteworthy and very difficult to explain in view of 

 the abundance of garnet in the rocks of Devon and Cornwall (for 

 instance, in the Lizard schists), in metamorphic aureoles round the 

 granites, and in contact-rocks near dykes and other intrusions, etc. 

 The difficulties are not decreased when it is remembered that the 

 mineral is abundant in Britanny, where it is associated with 

 staurolite, tourmaline, and other stress -minerals. 



Zircon and rutile are widespread in sediments, and are of little 

 determinative value as regards source of constituents. The origin 

 of the large quantities of rutile in sedimentary rocks is at present 

 unexplained. 



Anatase and brookite are interesting mineralogical curiosities, 

 but their wayward incoming and disappearance also supports the 

 view that they have little or no determinative value. 



Tourmaline, muscovite, andalusite, and topaz occur abundantly 

 in the Dartmoor and Cornish granites and other aureoles. The 

 presence of these minerals in large quantities in the sediments 

 here described might, therefore, be expected. On general evidence, 

 such as the presence of pseudostratitication indicated by the por- 

 pbyritic felspars, etc. of the uppermost layers of granite, it is 



1 Although it is always dangerous to reason from the absence of certain 

 minerals, the arguments are frequently not vitiated if the expression' of great 

 rarity ' be substituted for ' absence '. 



2 See, for example, R. H. Kastall & W. H. Wilcockson, Q. J. G. S. vol. lxxj 

 (1915-17) p. G20, table. 



