60 On the Use of the Pocket Box-Sextant. 



June 24, 1823. 



Ol. 1. 1st. obser. watch slow 9 8 

 2d 9 7 



It is needless to multiply instances of this nature ; they are 

 sufficient to shew that the above important element in longitude 

 calculation may be approximated to considerable exactness. 



The radius being barely 2 inches, they probably cannot be 

 divided more minutely than to 30" ; they might, however, be made 

 a little larger, if that were an object, and they would still possess 

 the advantages of having the mirror secured from accident and 

 derangement, which is perhaps a good reason for the constancy of 

 this index error in such small instruments ; but, we are now consi- 

 dering them as they are made at present by the best makers, and 

 with a telescope magnifying about four times ; although they could 

 carry one magnifying six or eight times, which would be still better. 

 It must be remembered, that an artificial horizon is absolutely 

 necessary on land, for such purposes as those last-detailed. 



Art. V. On the Concretionary and Crystalline Structures 

 of Rocks. By J. Mac Culloch, M.D., F.R.S., 8fc. 



[Communicated by the Avithor.] 

 This subject not only forms an important part of the natural 

 history of rocks ; but is also interesting, from the hints which it 

 affords respecting the causes which may have acted in their forma- 

 tions. These, and some other points, will be discussed in the course 

 of the follov.iug remarks. 



Of the Laminar, Foliated, and Schistose Structures. 

 The most important perhaps, if not the most conspicuous divi- 

 sion of structure, is that to which the term laminar may be applied. 

 This is the modification which has so often been confounded, under 

 some of its forms, with the stratified disposition ; giving rise, in the 

 cases of trap and granite, to serious errors. One of the most in- 

 teresting varieties in this division occurs in granite. The size of 

 the concretions, if such they are to be considered, is often im- 



