HG THE LECTURE ROOM, SMITHSONIAN INSTITUTION. 



been met with breaking througla these formations, and the relations 

 of the base of the lower silnrian group, along the foot of the hills 

 composed of the syenite, are such as to make it evident that the 

 fossiliferous beds in some places over-lie worn down parts of the 

 volcanic rock. But all these intrusive masses are cut by a set of 

 dykes, whose relations to the fossiliferous strata are not so certain. 

 These dykes are composed of a finely granular base, with an earthy 

 fracture, consisting of feldspar and pyroxene, and having a dark 

 brownish-grey color. In this base are imbedded rounded forms of 

 black augite, giving brilliant cleavage surfaces, and varying in size 

 from masses not bigger than a pin's head to some of several inches 

 in diameter. These are associated with various sized nodules of 

 <3alc-spar, filling cells that do not attain the diameter of the largest 

 masses of augite, and with small spangles of mica, grey in fresh 

 fractuces, but weathering to a brass-yellow on the surfaces of slightly 

 weathered cracks and joints. Small crystals of sphene were occa- 

 sionally observed in the rock. In the nomenclature of d'llalloy the 

 rock would be called u melaphyre, and is the augite-porpliyry of some 

 Grerman authors. By many geologists, from the accidental presence 

 of the calc-spar nodules, it would be called an amygdaloidal trap. 



These dykes bear a striking resemblance to some of those which 

 intersect the lower silnrian group in the vicinity of the mountain of 

 Montreal, and may be possibly of the same age ; but none of them 

 have yet been traced continuously from the Laurentian into the 

 fossiliferous rocks. 



"THE LECTURE ROOM, SMITHSONIAN INSTITUTION, 

 WASHINGTON. 



BY PROFESSOR HENRY, LL.D., WASHINGTON. 



Read before the Canadian Institute, February I3th, 1858. 



In the Eighth number of the Canadian Journal (Vol. II., p. 130), 

 was published an admirable paper, by Professor Henry, Secretary of 

 the Smithsonian Institution, on "Acoustics applied to public build- 

 ings." Through the kindness of the author, we are now enabled to 

 present to our readers the accompanying diagrams in illustration of 



