384 Correspondence — Lt.-Gen. C. A. McMahon — A. C. Seward. 



author writes': "I never said that chlorite is changed into biotite 



by contact-action only Chlorite is changed into biotite by 



contact action plus dynamic action." 



I will only say in conclusion, that I do not see that the illustra- 

 tions which Dr. Callaway has given in his July paper strengthen 

 his case. If I understand him, he seems to think (to put the 

 argument shortly) that because chlorite abounds where signs of 

 shearing exist that the hydrous chlorite has been produced by 

 shearing; and secondly, because he has observed cores of mica in 

 the chlorite, that the mica has been produced out of the chlorite 

 by contact-action. " The uniform appearance of mica," he writes, 

 " where the shearing is great and where the granite veins are 

 numerous, while it is nowhere seen where shearing and veins are 

 absent, appear to demonstrate that these are the true causes of the 

 generation of the mica." 



I think all the probabilities of the case are opposed to this view. 

 As I said in my last paper, all petrologists are ready to admit that 

 dark mica is a very common product of the contact-action of granite 

 intrusion in diorite ; and I think that the subsequent more or less 

 complete conversion of this contact mica into chlorite by aqueous 

 agents that have found ready access to the rock along the lines of 

 crushing and shearing is what one would naturally expect. That 

 there should be cores of mica left in the secondary hydrous chlorite 

 is in accordance with the petrologist's experience in his studies of 

 the conversion of olivine into serpentine and of augite into horn- 

 blende. Dr. Callaway's theory involves the supposition that the 

 production of hydrous chlorite should precede the gneiss of the 

 mica ; and that portions of the hydrous chlorite should escape 

 unsinged from the burning fiery furnace of contact metaraorphism 

 that converted their fellows into anhydrous mica. These, and other 

 difficulties enlarged on in my last paper, have not been met. 



C. A. McMahon. 

 [This correspondence is now concluded. — Edit. Geol. Mag.] 



CATALOGUE OP THE MESOZOIC PLANTS IN THE DEPARTMENT 

 OF GEOLOGY, BEITISH MUSEUM (NATURAL HISTORY). THE 

 WEALDEN FLORA, PART I. 1894.— A CORRECTION. 



Sir, — In adopting the generic name Nathorstia for a new type of 

 Wealden fern (" Wealden Flora," p. 145), I was not aware that the 

 late Prof. Heer had previously made use of the same genus. 



My thanks are due to Prof. Nathorst of Stockholm for calling my 

 attention to Heer's genus Nathorstia, which was instituted in 1880 

 for the reception of certain fragments of Marattiaceous ferns from 

 the Cretaceous strata of Pattorfik, Greenland (Flor. foss. Arct. 

 vol. vi. 1882. Nachtrage zur foss. flor. Gronlands, p. 5, pis. i. 

 and ii.). I propose, therefore, to substitute the generic name 

 Lechenbya for the fern described in the Catalogue as Nathorstia 

 vaklensis. 



Cambridge, July, 1894. A. C. SewaRD. 



1 Geol. Mag. July, 1894, p. 320. 



