156 Alfred Brammall — Intrusive Rock, 



Magnetite is abundant in some sections, and present in all. It 

 is titaniferous, as indicated by a greyish opaque outgrowth of 

 leucoxene. 



In more than a score of sections examined no augite or other 

 pyroxenic material was observed. Mica is wholly wanting. Apatite 

 is conspicuous as needles, from "025 mm. to '075 mm. in length, 

 embedded in the other constituents. Pyrites in granules up to 

 •3 mm. in length is also a notable accessory. 



5. Discussion of the Systematic Position of the several Varieties. 



Typically the felspar and hornblende show a marked tendency to 

 idiomorphism. The low degree of mutual interference shown by the 

 felspar prisms in particular, the almost total absence of the 'granitic', 

 and the prevalence of the ' gloraeroporphyritic ' structure are incom- 

 patible with diorites. On the other hand, these features may be 

 either hypabyssal or volcanic ; the absence of a glassy base eliminates 

 the latter as a probability, while the ophitic relationship of the 

 felspars and hornblende, and the basic character of the former 

 (labradorite), recall the structure of the typical dolerites, and the 

 coming in of olivine suggests analogy with the olivine dolerite type. 

 Dolerites, however, are typically pyroxenic, and in this rock pyroxenes 

 appear to be wholly wanting, and no secondary amphibole, which 

 might have been referred with reason to primary augite, was observed. 

 The idiomorphism and the primary character of the hornblende are 

 incompatible with the type dolerites. 



On the other hand, individual femic clots may monopolize the 

 whole of a field visible under the microscope ; in such case the 

 subordinate amount of the felspar visible, the serpentinous pseudo- 

 morphs of olivine, occurring as pcecilitic inclusions in the hornblende, 

 recall the picrites; but even this resemblance is illusive, for the field 

 adjoining such a femic patch may be entirely devoid of hornblende 

 and even comparatively free from its serpentinous decomposition 

 products. Moreover, the rock as a whole must be excluded from the 

 picrite group by reason of the high proportion of felspar and the very 

 small amount of olivine present. 



The idiomorphism and porphyritic aspect of the hornblende, 

 considered together with other hypabyssal features, admit the rock 

 without question into the lamprophyre group. The red-brown 

 hornblende, the essential absence of free silica, the conspicuous 

 apatite and titanium content link the rock with the type camptonite 

 of Campton Fells, N.H., and the classification of the main rock-mass 

 as a camptonite is open to little objection. The olivine, too 

 insignificant in quantity for the picrites, ranks as a noteworthy 

 accessory when the rock is regarded as a camptonite. In the typical 

 camptonite, however, hornblende recurs as microlites in the base*. 

 This is not the case in the Marston Jabet rock. 



The camptonite rock forms the bulk of any one of the sheets of 

 the intrusive mass. The olivine is more abundant in the centre of 

 the mass than near the margin, and it is in the centre that the 

 nearest approach to a picritic facies was observed. 



A partial analysis of the mottled rock was carried out in the 



