24 IGNEOtrS EOCKS OF SOUTH-EAST CORNWALL 



locality. Everything here, however, will of course depend upon 

 our ability to identify this special volcanic series by its lithologi- 

 eal characters. 



I do not believe that this is at all difficult, or that save in a 

 very few exceptional cases, any one who has studied these rocks 

 as a series, can fail to recognise them wherever they may be 

 found within the area under discussion. At least this is the 

 conclusion at which I have arrived from my own studies of these 

 rocks in the field, and from a careful microscopic examination of 

 a large number of selected examples. To get any useful re- 

 sults out of this line of evidence it is, however, essential that 

 the whole question shall be worked out de novo ; and the real 

 nature of our igneous rocks, mapped and unmapped, be clearly 

 ascertained. Especially is it essential to avoid framing any 

 conclusions from the nomenclature or identifications of our pre- 

 decessors. We must build to a large extent on their foundations, 

 but the materials must be our own, if our work is to have a^ny 

 current value. 



I append some notes on such of the igneous rocks of this 

 district as have been examined microscopically, with a view to 

 indicate their leading characters. 



The Lavas are commonly bluish- or greenish-grey in colour, 

 weathering rusty brown. An excellent amygdaloidal variety 

 from Landrake, largely quarried for road metal, is a bluish- grey 

 fine-grained rock with vesicles filled with calcite. The base con- 

 tains lath-shaped felspars with crystals of magnetite and pyrites, 

 grouped patchily, and a few microliths. The more distinct tuffs 

 consist chiefly of felspar granules, with a few quartz, and 

 occasionally finer interstitial matter. One of the most interest- 

 ing of this class is the soft schistose ochreous-drab rock which 

 crosses the Tamar at Saltash Bridge. This is made up of broken 

 felspar crystals and fragments of volcanic rock in a kind of 

 fluidal matrix. Some of the fragments are strongly outlined by 

 black streaks and strings from each other. The generally identi- 

 fiable minerals in this series are plagioelase-felspars and iron 

 oxides, with calcite fillings to vesicles ; any augite seems 

 to have disappeared. 



