148 MINERALOGY AND LITHOLOGY. 



Hampshire was Prof. O. P. Hubbard, of Dartmouth college, who in 1837 

 made a geological excursion through various parts of the state, and who 

 made special observations on the eruptive dykes.* This gentleman rec- 

 ognized clearly the great differences in these rocks, and noted the varia- 

 tions in their appearance, even when situated side by side. But the 

 means for careful discrimination being at that time beyond the reach of 

 our science, all these rocks were classed together as trap. I hope that 

 gentleman will not be displeased to know that some of his specimens, 

 collected so long ago, have fallen into my hands, and that we now have 

 the means for classifying them. 



Besides researches made on the rocks actually within our borders, 

 more extended studies have been prosecuted on allied rocks in adjoining 

 regions. The most prominent of the investigations are those prosecuted 

 by Dr. T. Sterry Hunt, when connected with the Canadian survey.f The 

 rocks which he studied and classified are nearly allied to ours, and in 

 many cases identical. In reference to this work it may be said, that al- 

 though microscopic study in many cases would, I think, cause a nomen- 

 clature essentially different to be adopted, yet any new results which 

 might by that means be attained would in no degree lessen the weight 

 of Dr. Hunt's reasoning in reference to the origin of this class of rocks, 

 or any of his theoretical and geological conclusions, which must be what 

 he chiefly values. The more essential features of Dr. Hunt's work have 

 been by him embodied in his Chemical and Geological Essays, a volume 

 easily accessible to all, while the geological report is not. 



All our basic eruptive rocks are essentially compounds of triclinic feld- 

 spar, with either hornblende or pyroxene, and, according to which of 

 these latter minerals is present, they are divided into two classes. For 

 the determination the microscope is often necessary, with the aid of 

 which a number of accessory minerals are found, which are either essen- 

 tial and constant, like magnetite and apatite, or accidental and variable, 

 like chlorite, biotite, calcite, etc. The pyroxene or hornblende has at 

 times been removed by process of alteration and decay ; but still its orig- 

 inal presence, and its influence in producing the structure observed, are 

 so plain as to make no difference in the classification of the rock. The 



* Am. your. Science, i, vol. xxxiv, p. 105. 

 f Geology of Canada, 1863. 



