Sorace B. Woodward — On the Metamorphism of Strata. 403 



into a smooth, hard, brittle, splintery rock, that might be called 

 Hornstone, and in which the fossils are still perfectly preserved. 

 This notice led us to seek Major-General Portlock- for particulars, 

 as it seems we have here a parallel case of change. It is the 

 calcareous clays of the Lower Lias, as seen at Ballintoy, that have 

 been altered into the Portrush rock. He gives some analyses made 

 by Dr. Aj^john, which roughly show the character of the change 

 to consist in a loss of carbonic acid and of a great deal of lime. 

 They prove it to be on the whole a molecular change rather than one 

 of replacement. 



We append an analysis given by De la Beche^ of the Eed Marl 

 at Aust, and which shows but little variation from the analysis of 

 the Ballintoy marls : — 



Ballintoy Marls. Portrush Rock. Eed Marls Aust. 



Silica 56-903 57-44 48-69 



Protoxide of Iron ■ 4-79 



Peroxide of Iron 7-43 10-16 9-09 



Alumina 1-76 23-48 8-77 



Lime 19-02* 5-94 8-68 



Magnesia 3-94* 2-68 0-94 



Carbonic acid 8-56 



Soda, Potash, etc 99-70- 623 



Water and loss 10-95 4-25 



100-00 100-02 



We have not met with any analyses of the Somersetshire Lias,' 

 but its general composition (the clays and limestones being taken 

 together) would doubtless be similar to that of these marls. Accurate 

 analyses of the beds would have been interesting, but they are not, 

 however, essential to the argument. 



Cotta^ mentions that in Germany the name of " Hornfels " is given 

 to certain rocks, the product of transmutation of argillaceous deposits, 

 found adjoining to plutonic rocks, to which they probably owe the 

 change they have undergone. 



Origin of Change. — We may, therefore, conclude that some similar 

 agency to that which has affected the Lias of Partrush has been 

 instrumental in altering the beds in Somersetshire. 



Unfortunately we have no traces of igneous rock at Harptree ; 

 indeed, the nearest point at which trap is exposed is distant about 

 eight miles, namely, at Wrington Warren. There are also dykes at 

 Downhead and Bleadon Hill. They do not seem to have effected 

 any great change on the rocks with which they are seen in contact, 

 nor does it appear that they were erupted at one period. 



The large basaltic dyke discovered by Mr. Moore,'' and found by 

 Mr. Ussher to extend from Tadhill House, Downhead, to near Long 



^ Pteport on the Geology of Londonderry, etc., pp. 97, 98, 140, 150, etc. 

 ^ Mem. Geol. Survey, vol. i. p. 254. 

 3 This is given as Matter insoluble in Hydrochloric acid. 

 * These are Carbonates. 



s Save one of the Lower Lias of Downside, near Shepton Mallet, the equivalent of 

 the Sutton Stone.— /*/«?, p. 276. 



^ Rocks Classified and Described. Edited by P. H. Lawrence, p. 350, 

 ' Quart. Journ. Geol. Soc, vol. xxiii., p. 451. 



