MAGNETIC OXIDE OF IRON. 87 



tities of the worn and rounded fragments cover tlie shore. It forms a beautiful material for 

 garden and door-yard walks, and ship loads of it might be transported to any part of the lake. 

 The examination of the beach formed of this material, e.\hibits a curious example of the 

 power of the waves to sort and collect the pebbles ; those, for instance, of a given size, are 

 gathered together, separated and placed by themselves. All the large ones occupy places 

 distinct from the smaller ; and in using them for walks, we have only to select the size we 

 wish for the object in view. 



There are no facts connected with the porphyry of Essex, by which its character as a 

 mineral bearing rock is revealed, except, indeed, those of a negative kind : no rock of this 

 character is found in connection with mineral veins or beds, in the northern counties. 



In giving the extent of this rock, I have supposed it to have been once a continuous mass, 

 and to have extended from Cannon's Point to Rattlesnake's den, a distance of about five miles. 

 This, however, is a matter of conjecture, and the opinion is founded upon the perfect simi- 

 larity of the rock at all the intermediate places where it is to be seen. 



I would take this occasion to recommend to students in geolog}-, to visit the shores of Lake 

 Champlain. It is a field full of interesting and instructive phenomena ; one in which the 

 dynamics of geology may be studied to the best advantage. Moreover, the field is quite 

 accessible, and every part may be visited at an expense not disproportionate to the advantages 

 which may be obtained. 



3. Magnetic Oxide of Irox. 



Reasons for placing the oxides of iron among the 7-ocks. — State of oxidation accidental. — 

 Form in which the two oxides occur among the rocks. — Masses and veins. — Appearance 

 of stratification. — Association of garnet with veins of 7nagnetic oxide, etc. 



On account of the great extent of the oxides of iron in the northern counties of New- York, 

 I have considered them deserving of a place among the rocks ; and as they resemble, in their 

 mode of occurrence, the trap, greenstone and porphyry, I have arranged them all together. 

 By this course, I do not mean to maintain that the resemblance is complete ; it is, however, 

 sufficiently near to admit of their being placed in juxtaposition. It will be perceived that I 

 have kept the two oxides of iron distinct ; though in this there appears to be no very good 

 reason, for the state of oxidation is to be regarded as an accidental circumstance, rather 

 than one which is fixed and necessary. It is true, undoubtedly, that the magnetic oxide is 

 formed of definite proportions of its elements originally, but it is liable to change, and pass 

 to the state of a peroxide ; to become, in fact, so far as its composition is concerned, the 

 specular oxide ; and it will probably appear, on a careful analysis of the magnetic ores, as 

 they exist in their beds, that some considerable diversity e.xists as it regards the proportion of 

 ojcygen in combuiation with the iron. Besides this, their mode of occurrence geologically is 

 not very different. The collective facts derived from both oxides as they exist in the earth, 

 when taken together, constitute but one class of phenomena, or bring out and establish but 



