RAMSAY AND EVANS. — OBSERVATIONS IN GEOLOGY. 7l 



phyries, diorites, &c. Are ^they pierced by trap dykes, passing 

 more or less across the planes of stratification as in Fig. 5, and 

 if so of what kinds. 



Fig. 5. 



Are common sedimentary strata ever associated with interbedded 

 lavas and volcanic ashes and tuffas, in such a way as to show that 

 they were poured and spread out under water at intervals during 

 the accumulation of the strata, and what is the mineral character 

 of the igneous rocks. In the case of lavas that have been poured 

 out over ancient sea bottoms, it may often be noted that the 

 sedimentary stratum underneath has been altered or baked by 

 the overlying melted mass of lava, while the sedimentary bed 

 that overlies the lava remains unaltered by heat, the underlying 

 lava having cooled before the deposition of the sediment that 

 succeeds it in the section exposed. This is one way to distinguish 

 between such lava beds, and sheets of melted matter that have 

 been forcibly injected between ordinary sedimentary strata. Beds 

 of coal, or of lignite, underlying sheets of igneous rocks, should be 

 examined to see whether any portion has become altered, possibly 

 into graphite. 



Fig. 6. 



Do igneous rocks, such as lavas and volcanic ashes, show 

 signs of having accumulated on land in successive layers. If so, 

 are there any signs of soils and plant-bearing beds between them, 

 or of other strata that may have been formed in fresh water, 

 bearing bivalve Crustacea, such as Cypris, &c., or any other kinds 

 of organic remains, such as fish, terrestrial mammalia, &c. &c. 



g. Besides strata that have merely been hardened into rocks 

 and show all the signs of ordinary stratification, it is probable 

 that tracts of metamorphic rocks may be met with, such as slaty 

 beds merely altered by slaty cleavage, also gneiss of various 

 kinds, mica schist, chlorite schist, hornblende schist, serpentines, 

 &c. For the theory of those alterations of common formations 

 that resulted in the production of metamorphic rocks still show- 

 ing traces of stratification, the observer must refer to any good 

 manual of geology. It is sufficient now that he should be able to 

 distinguish their leading varieties. 



It must be understood that gneiss and other metamorphic 

 rocks are not necessarily of the greatest geological antiquity. 



