318 Mr. D. Forbes's Researches in 



anxiety to occupy himself alone with what he would consider the 

 broader views of Nature's phenomena*. 



Just, however, as the mouse (in the old fable) proved of so great 

 assistance by nibbling away at the net in which the captive lion 

 was entangled, so in this instance also have we an example of 

 how useful the study of such minute details in mineralogical in- 

 vestigation may be when applied to the explanation of the geolo- 

 gical features of the district. 



The chemical and microscopical examination of specimens 

 of these rocks, taken from all the different eruptive bosses in 

 Staffordshire (thirteen in number) which are coloured as green- 

 stones on the map of the Geological Survey, as well as of many 

 specimens taken from intrusive masses or dykes met with in 

 depth in the coal-pits which do not make their appearance on 

 the surface, proved most conclusively that titanoferrite was inva- 

 riably present in all, and consequently that it is an essential 

 constituent of the rock itself. 



The chemical analysis and specific gravity of the mineral 

 further showed it to belong to the variety of titanoferrite which 

 usually accompanies the eruptive rocks of pakeozoic age, but 

 which is not as yet known in those of the later geological periods f. 



As these rocks, as before mentioned, invariably contain the 

 mineral species titanoferrite, it naturally follows that the pre- 

 sence of the comparatively rare chemical element titanium must 

 be equally characteristic of their intrusion or appearance, since 

 this element does not occur as a constituent of any other of the 

 rocks of this district J. By determining, therefore, the presence 

 of titanium, a means is at once obtained by which such rocks, 

 even when altered by metamorphic action so as to be unrecog- 

 nizable to the eye, can be distinguished, and by which also beds 

 or deposits (such as tuffs, ashes, clays, &c.) formed from the break- 

 ing up and alteration of such rocks might be referred to their 

 original source. 



To illustrate the application of this in practice, a couple of 

 instances may be cited. 



In September 1855 the occurrence of a deposit of dark-red 

 ferruginous clay in the outskirts of the town of Dudley, in ap- 



* And doubtless also staunch in the orthodox belief " that mountains 

 should not be looked at through microscopes." 



t Titanoferrite, when found in modern lavas and in the younger basaltic 

 rocks, occurs as iserine, monometric in its crystallization, and differing in 

 the proportions of its chemical constituents, from when it presents itself 

 as Ilmenite and other subspecies, which are rhombohedral and found in the 

 older rocks of palaeozoic age. 



X Titanium has been detected in some clay ironstones and clays in mi- 

 nute quantity, but under circumstances which could not mislead the ob- 

 server in such cases. 



