ESDLicH.] ACIDIC VOLCANIC ERUPTIVES — POEPH. TRACHYTE. 243 



was observed that the Palaeozoic beds were disturbed. Triassic and 

 Jurassic strata are more frequently affected. Lower Cretaceous sand- 

 stones are broken through, and are at many places altered into quartz- 

 ites. The largest amount of disturbance the Middle Cretaceous shales 

 have suffered. They have been distorted; their normal position has 

 been infringed upon by the trachytic lavas until both horizontal and 

 vertical displacements have occurred. Within the limits of the Middle 

 Cretaceous shales we find the greatest development of the trachytic 

 masses. Offering less resistance than underlying beds, partly from 

 their position, partly from their texture, they have been penetrated 

 largely by the lava. Mainly in an approximately horizontal direction 

 has this penetration taken place. To-day we fiod its evidence in the 

 huge sheets and wedges of porphyritic trachyte that interleave the 

 shales. 



At several localities Post-Cretaceous strata are traversed by dikes be- 

 longing to this group. They are not separable therefrom, either in the 

 character of their occurrence nor in their petrographic constitution. 



V. — INFLUENCE UPON SEDIMENTARY STRATA. 



The influence of the porphyritic trachytes upon sedimentary rocks is 

 dependent upon the primary effect produced thereupon. The question 

 referring to the great displacements of strata does not enter into con- 

 sideration at this place. We here consider only that produced by the 

 contact with each other. This may be : 



(1) Preserving, or 



(2) Destroying. 



From numerous observations it has been learned that the effect of the 

 hot lavas upon sedimentary rocks has been such as to metamorphose 

 them. This is accomplished in a very direct, uniform manner, but its 

 results differ somewhat. We find that by the action of heat sandstones 

 have been changed into hard quartzites, and even into aggregates 

 closely resembling granites. We find argillaceous, partly dolomitic 

 shales transformed into rocks that the experts would terai micaceous 

 schists. Shales and marls are altered into argillites and porcelain jas- 

 per. In case the cohesion of the entire mass is not destroyed, this pro- 

 cess has been a preserving one. Instances do occur, however, where 

 the unity of the mass has been broken up, where the result of heat upon 

 the strata was such as to render them more effectually attacked by erod- 

 ing agents. This, In one sense, may be characterized as destruction of 

 the strata. 



I have in view, however, still another process by which the destruc- 

 tion of originally unchanged sedimentary rocks is accomplished. This 

 is due to the action of fusion. The products thereof I name 



SYMMORPniO* ROCKS. 



In passing through the fissures in sedimentary strata, the lavas of this 

 trachytic group have frequently enveloped fragments thereof of more or 

 less weight, and have^carried them along. Such fragments can often 

 be found in a metamorphosed condition. Taking the case that they 

 were small, though numerous perhaps, it is easy to see that they might 

 be entirely assimilated by the hot lava inclosing them. This would 



*NoTE. — I have coined this word to express what is described in the subjoined page. 

 It seems essential, and is certainly more convenient for descriptive purposes, to sepa- 

 rate so interesting a group by a definite, expressive term. — E 



