10 SHALER ON THE CLASSIFICATION OF LAVAS 



miles, where every square mile would give several examples of such injections. In the 

 older rocks of New England these dykes generally exceed a dozen in number to the square 

 mile, and often rise to hundreds in such areas. Yet there are considerable areas of Archaean 

 rocks, where such injections are very rare. The results of my personal enquiries into the 

 origin and distribution of dykes are briefly stated in the following propositions, viz : 



1. Dykes or enclosed lavas are only formed when rocks have been buried by sub- 

 sequent accumulation to considerable depths beneath the surface, metamorphosed 

 during such burial, and also subjected to the action of mountain-building forces. 



A glance at the section from the Atlantic Coast in North Carolina westward into the cen- 

 tre of the Mississippi Valley, or from the Laurentian Hills southward to the Ohio, will show 

 the peculiar differences in the frequency of their occurrence which may be traced in rocks 

 of different ages. Whenever the section passes through the old Archaean rocks we find 

 a very great abundance of dykes, the injections distinctly showing their igneous origin. 

 When we pass above the level of the Archaean and enter into the lowest Cambrian section 

 these intrusions disappear, and veins, i. e., deposits formed by the filling of fissures through 

 water action abound. I have never seen a single dyke in the Cambrian rocks of East 

 Tennessee or Southwestern Virginia, though they are extremely effected by mountain 

 dislocations and cut by the deep faults that traverse every part of our Eastern American 

 rocks. There are some of these faults that give a vertical throw of over ten thousand 

 feet, and must penetrate to very great depths in the crust, yet in no case have they 

 afforded a passage to lavas. Within the limits of the State of Kentucky, an area of about 

 40,000 square miles, I am satisfied that there is not a trace of dyke injection, though vein 

 stones with contents that require us to suppose that they have been deposited by heated 

 waters abound there. I am not aware of the existence of a single dyke within a radius 

 of two hundred and fifty miles from the city of Cincinnati, though the geology of this 

 region is pretty Avell known to me. 



In all these regions where dykes abound, the metamorphic character of the rocks enables 

 us to make sure that they have been subjected to extensive alteration from heat, and in 

 most if not all cases this heat has been brought into the strata by the protective effect of 

 thick sections, that rested on the surface at the time the dykes were formed, but have since 

 been eroded. Moreover, in all cases the regions extensively traversed by dykes have been 

 greatly disturbed by mountain-building forces. We search in vain for horizontally dis- 

 posed rocks penetrated by dykes or for recent beds that are marked by such injections. 



2. Dyke stones are injected into fissures formed by the rupture of beds through 

 contractions due lo metamorphosis, or to the contortion of beds, and their mate- 

 rials represent the r.iore siliceous deposits of the subjacent rocks of the section. 



The formation of the fissures which give rise to veins and to dykes is clearly due to 

 allied causes. Considered as fissures, they are all clearly to be put into the same class. 

 They are both formed by strains in the rocks ; they both first exist as openings, into 

 which their contents are brought either slowly by the action of water, or rapidly through 

 the action of igneous forces. Although we never find gash veins, i. e., fissures that only 

 have a limited extension downwards, filled by dyke stones, we find true fissures, or those 



