598 VOLCANIC ROCKS. 



There is reason at least to suppose that a great 

 many of these rocks have their counterparts in the 

 volcanic productions. But as no positive demon- 

 stration on this subject could be procured, it was 

 thought advisable to model the catalogue by the 

 writings of those who have had the greatest expe- 

 rience, and who seem to have laboured under the 

 fewest prejudices. Some alterations, which were 

 suggested by the examination of specimens, by 

 comparing the reports of others, and by general 

 analogies, have been made ; but the whole must 

 remain for a correction which, it is hoped, is not 

 far distant 



It may only be remarked, in concluding this 

 obscure subject, that if a general modification of 

 the whole Synopsis should be required, it will 

 probably consist in a different manner of treating 

 those substances which are here enumerated only 

 as imbedded minerals. The quantity of Leucite, 

 for example, which occurs in many of the lavas, 

 is such that it forms a principal part of the rock, 

 and must perhaps therefore be considered essen- 

 tial, not accidental. Whenever a complete collec- 



