Reports on the Geology of the State of Maine. ' 147 



beds of blue magnesian limestone, the compact rock, being al- 

 ways transformed into granular and semi-crystallized dolomite ; 

 the extent of the change being exactly proportionate to the size 

 of the dykes." " It is interesting to observe," says Dr. Jackson, 

 " that the most valuable quarries of limestone opened, are those 

 distinguished by the dykes ; and even the lime burners, who are 

 certainly not aware what opinions are entertained by geologists, 

 and cannot be accused of theoretical bias, attribute a good influ- 

 ence to the presence of this rock." The opinion of Von Buch, 

 that dolomite, or magnesian limestone, owes its magnesia in some 

 way to the igneous influence of the trap dykes which are now 

 found intersecting it, is not confirmed by any facts at the locali- 

 ties here referred to, nor indeed by any, so far as we know, which 

 have been brought to light in this country ; and Dr. Jackson 

 hesitates not to say, " that after carefully examining the places 

 referred to in Europe by Yon Buch, he is convinced, that the 

 igneousrocks acted there also, only by fusing a limestone which 

 contained magnesia in the original state." 



Rich veins of magnetic iron ore, apparently contemporaneous 

 with the dykes of trap, were discovered in the granite and mica 

 slate of Mount Desert and Marshall's Island. They are the most 

 compact variety, (the oxide ferroso ferricum of Berzelius,) and 

 possess polarity to' a most astonishing degree ; for Dr. Jackson ob- 

 serves, that a "compass needle will not traverse within thirty or 

 forty yards of a vein ; and when a crowbar, or drill of steel, is 

 struck upon its surface, it instantly becomes a strong magnet, and 

 when suspended by a cord, will oscillate, and swing to the north 

 like a compass needle." 



The connection which exists between these dykes and the va- 

 rious metallic ores mentioned in the reports, is not a little remark- 

 able, and throws much light on the origin of both. It is thus 

 spoken of in the first report. 



" It will be remarked by those who examine the facts stated in this 

 Report, that all the metallic ores which have been described, are found 

 in those places where trap dykes have been thrown up ; and no one can 

 doubt that their origin was in some way connected. This fact is not 

 only interesting in a theoretical point of view, but also offers a vahiable 

 guide to those who are seeking to discover metalliferous veins. It is also 

 an indication that the various ores mentioned, were injected or sublimed 

 from below, and that the veins may probably widen, and improve as 



