48 Geological Society. 



Dechen, Oeynhausen, and a host of geologists, which indicates the 

 posterior intrusion of such veins as branch upwards from large 

 bodies of granite, and ramify in thin filaments through the overlying 

 killas. If, however, granitic veins be formed by segregation, and 

 if the masses of schist within a granite vein are but portions of that 

 vein under a different state of development, by what happy accident, 

 we would ask, do the angles of the entangled fragments accord with 

 each other, or with those of the wall of the vein itself? Concre- 

 tions with some approximation to regular forms, may have sepa- 

 rated themselves chemically from mineral masses to which they are 

 subordinate : but no mode of chemical action can offer us an intel- 

 ligible explanation of the angular fragments of killas imbedded in the 

 granite veins of Trewavas Head and other parts of Cornwall. They 

 can be explained rationally only on the supposition of the mechanical 

 protrusion of the vein itself into the mass of the pre-existing slate- 

 rock. But Dr. Boase extends the horizon of his speculations : he will 

 not even concede that the dykes of porphyry and elvan in Cornwall 

 afford any proofs of igneous origin or of subsequent intrusion ; and 

 rising with his favourite hypothesis into hostility to every operation 

 of fire, he at length avows himself sceptical as to the volcanic origin 

 of all trappaean rocks. Whatever may be the value of the arguments 

 applied to Cornwall, they must be seriously weakened by a mode of 

 reasoning which compels the author to deny the existence of phe- 

 nomena which may really be considered as mathematically demon- 

 strated, and which are now registered among the fixed principles of 

 the science. If Cornwall does not offer clear proofs of the igneous 

 origin of any of its rocks to the eye of Dr. Boase, why does he not 

 seek to verify or reject his theoretical inductions by an appeal to 

 countries where the evidences are clearer? Let him visit Scotland, 

 and inspect all those appearances of intrusive granite, which have 

 long since been recorded by the approving testimonies of Hutton, Hall, 

 Playfair, and Seymour ; and let him follow MacCulloch through the 

 Hebrides, and then inform us whether he has not been led to a modi- 

 fication of his views. But if these evidences should not produce the 

 expected result, I would urge him to travel into central France, where 

 in one limited district is seen a succession of epochs of igneous 

 intensity, from the oldest trappaean rocks to modern basaltic lavas, 

 demonstrating that all these have originated in a succession of similar 

 volcanic causes. Notwithstanding, however, the discrepancy between 

 the views of Dr. Boase and those of most modern observers, i repeat 

 that his work being the result of long-continued examination, is well 

 worthy of your study from the valuable facts which it presents. 



In the mean time, permit me to notice how the evidences of for- 

 mer volcanic agencies have lately been operating upon the minds of 

 observers in distant parts of the world. In a recent work by Messrs. 

 Jackson and Alger upon the Geology and Mineralogy of Nova Scotia, 

 the authors acknowledge that in the commencement of their survey 

 they were biassed in favour of the Wernerian theory j but when they 

 met with repeated instances of sedimentary deposits, suddenly al- 

 tered and rendered crystalline, and of vegetable remains charred 



